30 Hidden Buddha the 30 Days of the Month (from the A-to-Z Japanese Deity Dictionary).

This set of thirty Buddha and Bodhisattva protect the thirty days of the month -- one for each day of the 30-day lunar month. However, this grouping does not appear in Buddhist scriptures. Scholar Fabio Rambelli

REFERENCES:

Professor Fabio Rambelli (University of California, Santa Barbara). See his Secret Buddhas: The Limits of Buddhist Representation. in Monumenta Nipponica, Vol. 57, No. 3, Autumn, 2002, pp. 271-307.

On page 278, Rambelli writes: "Another possible source of ideas and practices concerning hibutsu may be the Chinese idea of thirty "secret Buddhas" (mifo, Jp. = hibutsu) that were thought to protect the days of the month. The set included popular buddhas, bodhisattvas, and heavenly deities. See Bukkyo Daijiten, Vol. 2, p. 1560. This belief, found in China already in the 4th century, spread in the medieval period to Japan, where the thirty deities were incorporated into a cult of thirty guardian deities (sanjubanjin), one for each day of the month, that was particularly important in the pre-modern Nichiren sect (on the sanjubanjin, see Dolce, pp.222-54). It is conceivable that the connection of the Chinese secret buddhas with specific days could have influenced periodic displays in Japan of hibutsu in particular years, days, and months. As a further indication of a possible Chinese origin (or, at least, of the fact that some Japanese Buddhists thought hibutsu practices had a Chinese origin), a mid-Edo text, Shinzoku Butsuji Hen, argues that displays of hidden images at thirty-year intervals were already occurring during the Tang dynasty (the text specifically mentions a display in 818); see Shinzoku Butsuji Hen, p. 23." <end quote>

believes the concept of thirty "secret Buddhas" (Chn. = mìfó 秘佛; Jp. = hibutsu 秘仏) can be traced back to 4th century China, although most Japanese scholars believe it originated in China during the Five Dynasties Ten Kingdoms Period 五代十国時代 (907-960) and spread from there to Japan. This grouping is commonly attributed to Kai Zenshi 戒禅師 (aka Shikai Zenshi 師戒禅師), a 10th-century Chinese monk living amidst the famed Zen monastic centers at Goso-zan 五祖山 (lit. "Mountain of the Five Zen Patriarchs;" aka Mount Shuangfeng 雙峰) located in China's modern-day Hubei Province. It was on this twin-peaked mountain that Daoxin 道信 (Jp. = Dōshin; 4th Chan patriarch; 580–651) and Hongren 弘忍 (Jp. = Kōnin; fifth Chan patriarch, 601-675) established their centers of practice, with Daoxin's on the western peak and Hongren's on the eastern. The term Tōsan Hōmon 東山法門 (Chn. = Dōngshān Fǎmén) refers to the teachings of both. <sources here

REFERENCES: My sources for the origin of the 30 Buddhist Deities of the 30 Monthly Days come from:

Special Edition of the Rekishi Tokuhon Rinji Zokan entitled "Manyu Koyomi Hyakka," ed. Wakamori Taro (1915-1977), 444 pages, published by Shin Jinbutsu Oraisha (Tokyo), December 1973. See article by scholar Kitagawa Chikashi (1911-1986), which says the 30 Butsu of the 30 Days originated with the monk Kai Zenshi in present-day Hubei Province (China) at the Zen mountain enclave known as Goso-zan.
Butsuzo-zu-i, the "Collected Illustrations of Buddhist Images," 2005 reprint with commentary by Ito Takemi (b. 1927), pages 268-287. Modern-day reprints of the expanded 1886 Meiji-era version, with commentary by Ito Takemi, are available at this online store (J-site). Ito also attributes the origin of this grouping to monk Kai Zenshi at Goso-zan in China.
Mochizuki Bukkyo Daijiten (MBDJ), ed. Mochizuki Shinko, 10 vols. enlarged and corrected edition. Tokyo. Sekai Seiten Kanko Kyokai (1933–1936). Page 1560. Mentions a 10th-century Chinese grouping of 30 Buddhist deities called Sanjunichi Butsu-myo.
Digital Dictionary of Buddhism, Tosan Homon. Sign in with user name = guest. The DDB provided background information about Goso-zan.
For more details on above references, plus Japanese spellings, see the LEARN MORE section of this page.

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A few centuries later the grouping began to appear in Japanese texts, including the 14th-century Kokanzenji-roku 虎関禅師録 (Records of Zen Teacher Koganshiren 虎関師練; 1278－1346; a Rinzai-sect monk) and the mid-15th-century text Gaun-nikken-roku 臥雲日件録, a diary by Rinzai-sect monk Zuikei Shūhō 瑞渓周鳳 (1392-1473). <source

REFERENCE: Commentary by Ito Takemi (born 1927), pages 268-268, in the 2005 reprint of the Butsuzo-zu-i (Collected Illustrations of Buddhist Images; first published 1690). Modern-day reprints of the expanded 1886 Meiji-era version, with Ito's commentary, are available at this online store (J-site).

> This is most curious, as Japan's Zen sect typically rejects reliance on the myriad Buddhist deities and instead focuses on only a handful of divinities (e.g., the Historical Buddha, the Arhats) and stresses the great importance of developing a heart-mind connection with a living teacher. (Thus, I am surprised by the origins of this grouping in Chan / Zen circles.)

This imported set of 30 Buddhist deities may have sparked the development of a similar set of 30 Kami (Indiginous Japanese Deities) of the 30 Days. This latter kami grouping appeared around the 11th century in Japanese records, but did not come to prominence until the Muromachi period (14th and 15th centuries). The 30 kami in this latter grouping, moreover, are unequivocally paired with Buddhist counterparts in a combinatory Buddha-Kami matrix known as Honji Suijaku

NOTES ON HONJI SUIJAKU. During Japan's Heian era (794 - 1185), the numerous kami (Shinto deities) were recognized as traces or manifestations or incarnations (suijaku) of the Buddhist divinities (honji or honjibutsu), and a great syncretic melding occurred, with shrines and temples sharing both deities and sacred grounds.

The Tendai shrine-temple multiplex on Mt. Hiei is a prime example of the syncretic merging of Buddhist and Shinto deities in Japan. The idea of KAMI as Gohojin (guardian deities of the Buddhist doctrine) was a common element in the Heian period. This Kami-Buddhist syncretism was actually formalized and pursued based on a theory called Honji Suijaku, with the Buddhist deities regarded as the honji (original manifestation) and the Shinto kami as their suijaku (incarnations). Another similar term denoting the association between Buddha and Kami is Shinbutsu Shugo. By the late 12th century, the Mt. Hiei shrine-temple complex formed a well developed honji suijaku complex, or syncretic merger of both Buddhist and Shinto practices and deities.

Honji Suijaku was originally a Buddhist term used to explain the Buddha's nature as a metaphysical being (honji) and the historical human figure Sakyamuni (suijaku) as the manifest trace. In Japan's early Nara period, the honji were regarded as more important than the suijaku. Gradually they both came to be regarded as one, with neither the honji or suijaku considered more important. By the 9th century, Buddhist temples were constructed alongside Shinto shrines on many sacred mountains, epitomized by the holy Shugendo places throughout the Yoshino mountain range and by the powerful Tendai multiplex on Mt. Hiei. The native Shinto kami (deities) residing on these peaks were considered manifestations of Buddhist divinities, and pilgrimages to these sites were believed to bring double favor from both their Shinto and Buddhist counterparts. Another major center of syncretism was the Kasuga Shrine in Nara. The number of deities proliferated. Despite earlier resistance, syncretism was relatively smooth and marked by religious tolerance.

The Honji Suijaku theory was used for most of Japan's religious history to explain the relationship between the kami (indiginous gods) and imported Buddhist divinities. It was only after the Meiji Period, when the government forceably separated the two camps, that Buddhism and Shintoism were portrayed as two distinct philosophies.

本地垂迹. However, the Buddha-Kami pairings show no known correlation. The two lists, therefore, should be considered distinct, the first wholly Buddhist in nature, and the second (the kami list) combinatory. Even more confusing, images of these Buddha-Kami pairings appear in the 1690 Butsuzō-zui -- but the kami images include descriptive text that gives yet another Honji Suijaku 本地垂迹 (Buddha-Kami) pairing. Yet, both lists seem to have developed in tandem, and future research might find a common connection. Based solely on extant artwork, the 30-kami set appears to have attained much broader appeal, although the Hibutsu (Secret Buddha) set still serves today as the primary source for the ENNICHI 縁日 (holy days) of these Buddhist deities. See sidebar above.

Secret Buddhist Deities of the 30 Days of the Month Sanjūnichi Hibutsu 三十日秘仏, Source Butsuzō-zui
CLICK IMAGE TO ENLARGE Sanjūnichi Hibutsu 三十日秘仏 The Secret (Hidden) Buddha of the Thirty Days; as appearing in the Butsuzō-zu-i 仏像図彙, the "Illustrated Compendium of Buddhist Images." First published in 1690 (Genroku 元禄 3). Above images compiled and translated by Mark Schumacher from the expanded 1783 version. Both the 1690 and 1783 versions include this grouping, although some of the images are different. This set of deities includes some of Japan's most popular Buddha and Bodhisattva, but it also includes little-known deities from the Lotus Sutra and three Buddhas of the Past (Jōkō Butsu, Tahō Butsu, & Daitsūchishō Butsu) -- these last three come from a vast number of Buddhas of the Past, but most of these Past Buddhas are largely ignored or neglected in Japan & mainland Asia.
LIST OF THIRTY BUDDHIST DEITIES OF THIRTY DAYS Saying prayers to the deity on his/her specific day is believed to bring greater merits and results than on regular days.
Day	Buddhist Deity	Japanese Reading	Sanskrit Spelling, Description, & Links
1	定光佛	Jōkō Butsu Joko Butsu Buddha of the Past	Dīpaṃkara Buddha, deity of eternal light. In Japan, also known as Nentō Butsu 然燈佛. The 24th predecessor of Śākyamuni (Historical Buddha), who appears during the Buddha's preaching of the Lotus Sutra. Also translated as 燃燈佛 and 錠光; transliterated as 提洹竭, 提和竭, 提洹羯; 大和竭羅. See T 1428.22.783b10. <source: DDB login = guest>
2	燈明佛	Tōmyō Butsu Tomyo Butsu	Candrārkadīpa, the Buddha of Illumination; appears in the Lotus Sūtra T 262.9.4a8. Full name is 日月燈明佛, or Buddha of the Illumination of the Sun and Moon. <DDB login = guest>
3	多寶佛多宝仏	Tahō Butsu Taho Butsu Buddha of the Past	Prabhūtaratna Buddha. Buddha of Many Jewels. A Buddha of ancient ages, written about in the Lotus Sutra (Chapter 11), who emerged one day from ecstasy to invite Sakyamuni (the Historical Buddha) to share his throne and glory. <Flammrion, p. 94>
4	阿閦佛	Ashuku Butsu	Akṣobhya Buddha. Details Here.
5	弥勒仏・彌勒佛	Miroku Butsu	Maitreya, Buddha of the Future. Details Here.
6	二萬燈明佛	Nimantōmyō Butsu Nimantomyo Butsu	Buddha of Twenty Thousand Illuminations; appears in the Guanxinlun shu 觀心論疏 (T1921_.46.0587b18), a Chinese text by Guanding (561–632).
7	三萬燈明佛	Sanmantōmyō Butsu Sanmantomyo Butsu	Buddha of Thirty Thousand Illuminations, a deity who appears in the Keiran Shūyō Shū (溪嵐拾葉集, T2410_.76.0711c27), a text compiled between 1311 & 1348 by the monk Kōshū 光宗.
8	薬師如来	Yakushi Nyorai	Bhaiṣajyaguru Tathāgata, Medicine/Healing Buddha. Details Here.
9	大通智勝佛	Daitsūchishō Butsu Daitsuchisho Butsu Buddha of the Past	Mahābhijñābhibhū, Buddha of Supreme Penetration & Wisdom. A Buddha of ancient ages appearing in the Móhē zhǐguān 摩訶止觀 T 1911.46.130b27, a major text of the Tendai school (ca 594 AD). His sixteen sons include Amitâbha (Jp. = Amida; his ninth son) and Śākyamuni (Historical Buddha, his 16th son), and many of Buddhism's practitioners in our current age are said to be reincarnations of those who were his long-ago disciples (see Lotus Sutra, Chapter 11). <DDB; login = guest>
10	日月燈明佛 lotus sutra	Nichigetsu Tōmyō Butsu Nichigetsu Tomyo Butsu	Candra Sūrya Pradīpa Buddha; also called Candrārkadīpa, a deity with the brilliance of the sun, moon, and lamps; mentioned in the Lotus Sūtra 法華經 T 262.9.4a8. <DDB; sign in with user name = guest>
11	歓喜佛歓喜天	Kangi Butsu or Kankiten, Kangiten In Japan, more properly classified as a Deva (Jp. Ten or Tenbu)	Nandikêśvara. Other names include Ganesa, Ganapat, or Vinayaka. Depicted with two elephant heads with human bodies, one of which is Avalokitêśvara (Kannon).
12	難勝佛	Nanshō Butsu Nansho Butsu	Sudurjayā Buddha. Nanshō 椝勝 is the fifth of ten stages of the Bodhisattva path, and represents the stage where the Bodhisattva has overcome the worst difficulties. Appears in Bussetu Butsumyōkyō 佛説佛名經 T 0440_.14.0114b05
13	虚空蔵菩薩	Kokūzō (Kokuzo) Bosatsu	Ākāśagarbha. Details Here.
14	普賢菩薩	Fugen Bostsu	Samantabhadra. Details Here.
15	阿弥陀佛	Amida Butsu	Amitābha. Details Here.
16	陀羅尼菩薩	Darani Bosatsu	Dhāraṇī; one with great powers to protect and save. Appears in the Fan-fan-yu 翻梵語 T 2130.54.992a15 (6th century)
17	龍樹菩薩	Ryūju Bosatsu Ryuju Bosatsu	Nāgârjuna. A real person from India (2nd-3rd century) and one of the most lauded figures in Buddhist history, considered by many as the greatest debater, thinker, and writer on Buddhism. <DDB; sign in with user name = guest>
18	観世音菩薩	Kanzeon Bosatsu	Avalokitêśvara. Details Here.
19	日光菩薩	Nikkō (Nikko) Bosatsu	Sūryaprabha. Details Here.
20	月光菩薩	Gakkō (Gakko) Bosatsu	Candraprabha. Details Here.
21	無盡意菩薩	Mujini Bosatsu	Akṣayamati. Real monk and translator from China (active 4th-5th century). <DDB login = guest>
22	施無畏菩薩	Senmui Bosatsu	Bestower of Fearlessness; one of Avalokitêśvara's (Kannon's) titles.
23	大勢至菩薩	Daiseishi Bosatsu	Mahāsthāmaprāpta. Details Here. Bodhisattva of Wisdom, appearing often with Kannon in Japanese artwork called the Amida Triad.
24	地蔵菩薩	Jizō (Jizo) Bosatsu	Kṣitigarbha. Details Here.
25	文殊菩薩	Monju Bosatsu	Mañjuśrī. Details Here.
26	薬上菩薩	Yakujō (Yakujo) Bosatsu	Bhaiṣajyasamudgata. Details Here.
27	盧遮那佛	Roshana (Rushana) Butsu	Vairocana. Cosmic/Sun Buddha. Details Here.
28	大日如来	Dainichi Nyorai	Vairocana. Cosmic/Sun Buddha. Details Here.
29	薬王菩薩	Yaku-ō (Yaku-o) Bosatsu	Bhaiṣajya-rāja. Details Here.
30	釈迦如来	Shaka Nyorai	Śākyamuni, Historical Buddha. Details Here.
31	In Japan, the lunar calendar was abandoned in 1872 in favor of the solar (Gregorian) calendar. Some Japanese temples and shrines have since added a 31st deity to reflect the 31-day month. There seems to be no rhyme or reason to the varying deities so employed.

Other Titles / Translations for This Set of Thirty Buddha Names of the 30 Days of the Month (Sanjūnichi Butsumyō 三十日佛名). <trans. by Lucia Dolce, Buddhas & Kami in Japan. Honji Suijaku as a Combinatory Paradigm, 2003, p. 226. Esoteric Buddhas of the Thirty Days. <trans. by Anita Khanna, Jawaharlal Nehru University, New Delhi, Buddhist Iconography in the Butsuzōzuii of Hidenobu, 2010, pp. 87-96). Thirty Buddhist Tutelaries of the Thirty Days of the Month. <trans. Mark Schumacher>

SOURCES & ADDITIONAL REFERENCES

Butsuzō zui 仏像図彙 (Illustrated Compendium of Buddhist Images). Published in 1690 (Genroku 元禄 3). A major Japanese dictionary of Buddhist iconography. Hundreds of black-and-white drawings, with deities classified into categories based on function and attributes. For an extant copy from 1690, visit the Tokyo Metropolitan Central Library. An expanded version, known as the Zōho Shoshū Butsuzō-zui 増補諸宗仏像図彙 (Enlarged Edition Encompassing Various Sects of the Illustrated Compendium of Buddhist Images), was published in 1783. View a digitized version (1796 reprint of the 1783 edition) at the Ehime University Library. Modern-day reprints of the expanded 1886 Meiji-era version, with commentary by Ito Takemi (b. 1927), are also available at this online store (J-site). In addition, see Buddhist Iconography in the Butsuzō-zui of Hidenobu (1783 enlarged version), translated into English by Anita Khanna, Jawaharlal Nehru University, New Delhi, 2010.
Dolce, Lucia (2003) Hokke Shinto. Kami in the Nichiren Tradition. In: Teeuwen, M. and Rambelli, F.,
(eds.), Buddhas and Kami in Japan. Honji Suijaku as a Combinatory Paradigm. Curzon Routledge, pp. 222-54.
Origin of the 30 Hidden Butsu of the 30 Days of the Month. My sources all come from modern Japanese references, each telling essentially the same story:

Special Edition of the Rekishi Tokuhon Rinji Zōkan 歴史読本臨時増刊 entitled Manyū Koyomi Hyakka 万有こよみ百科, ed. Wakamori Tarō 和歌森太郎 (1915-1977), 444 pages, published by Shin Jinbutsu Ōraisha 新人物往来社 (Tokyo), December 1973. In the article by Kitagawa Chikashi 喜多川周之 (1911-1986), he says the 30 Butsu of the 30 Days originated with the monk Kai Zenshi 戒禅師 in present-day Hubei Province (China) at the Zen mountain enclave known as Goso-zan 五祖山. <See NACSIS listing, or purchase publication online from the Japanese Association of Dealers in Old Books.
Butsuzō-zu-i 仏像図彙, the "Collected Illustrations of Buddhist Images," 2005 reprint with commentary by Itō Takemi 伊藤武美 (b. 1927), pages 268-287. First published in 1690 (Genroku 元禄 3), the Butsuzō-zu-i is one of Japan's most influential studies of Buddhist iconography. Modern-day reprints of the expanded 1886 Meiji-era version, with commentary by Itō Takemi, are available at this online store (J-site). Ito gives Goso-zan as the origin of the grouping, and lists other sources from the 14th and 15th centuries.
Mochizuki Bukkyō Daijiten (MBDJ) 望月佛教大辭典, ed. Mochizuki Shinkō 望月信亨, 10 vols. enlarged and corrected edition 増訂版. Published by Sekai Seiten Kankō Kyōkai (Tokyo; 1933–1936). Vol. 2, Page 1560. Mentions the Chinese set called Sanjūnichi Butsumyō 三十日佛名 (Buddha Names of the Thirty Days of the Month).
Rambelli, Fabio. Secret Buddhas: The Limits of Buddhist Representation. Monumenta Nipponica, Vol. 57, No. 3, Autumn, 2002, pp. 271-307. On page 278, Rambelli writes: "Another possible source of ideas and practices concerning hibutsu may be the Chinese idea of thirty "secret Buddhas" (mifo 秘佛, Jp. = hibutsu 秘仏) that were thought to protect the days of the month. The set included popular buddhas, bodhisattvas, and heavenly deities. See Bukkyō Daijiten, Vol. 2, p. 1560. This belief, found in China already in the 4th century, spread in the medieval period to Japan, where the thirty deities were incorporated into a cult of thirty guardian deities (sanjūbanjin 三十番神), one for each day of the month, that was particularly important in the pre-modern Nichiren 日蓮 sect (on the sanjūbanjin, see Dolce, pp. 222-54). It is conceivable that the connection of the Chinese secret buddhas with specific days could have influenced periodic displays in Japan of hibutsu in particular years, days, and months. As a further indication of a possible Chinese origin (or, at least, of the fact that some Japanese Buddhists thought hibutsu practices had a Chinese origin), a mid-Edo text, Shinzoku Butsuji Hen 真俗仏事編, argues that displays of hidden images at thirty-year intervals were already occurring during the Tang dynasty (the text specifically mentions a display in 818); see Shinzoku Butsuji Hen, p. 23." <end quote>
Origin of Ennichi (Holy Days) ffortune.net/calen/calen/yomi99/yomi057.htm

Siebold, Philipp Franz von (1796-1866). Nippon Archiv zur Beschreibung von Japan, Leiden (1831 CE). All Images | Image Appearing Above.
Tobibudo's List of All 30 Secret Buddha 三十日秘仏, their holy days, and their Sanskrit seeds. Images and seeds from the Butsuzō-zu-i 仏像図彙. (Japanese only).
Thirty Butsu of the Thirty Days in Pictures 写仏の手本三十日秘仏. This 78-page book by Maki Yūkei 牧宥恵 (born 1950) presents descriptions and black & white line art. No color photos. Published 2004. Purchase book at Amazon. (I have not read this book, but offer it here simply as a note to myself to investigate later).
Calendar with One Buddha Per Day. Purchase online. Visit page, then select the calendar named 一日一仏＜日めくりカレンダー＞. This 31-page calendar includes 30 black and white drawings of the 30 Butsu of the 30 Days of the Month. J-site.
Example of Incorrect List. Many Japanese web sites (e.g., see this example) incorrectly link the 30 Butsu of the 30 Days (this page) with the 30 Kami Tutelaries of the 30 Days. However, the latter kami grouping is associated with a completely different set of 30 Butsu (see 30 Kami page for details and the correct associations).