What explains the membership of Shingon's 13 Buddhas list? The list of 13 Buddhas (which is actually Buddhas, Bodhisattvas, and Wisdom Kings) plays a role in Shingon gongyo and funerary rites.  What accounts for why these particular 13 are set aside for special attention?


Fudo (Acala)
Shaka (Sakyamuni)
Monju (Manjushri)
Fugen (Samantabhadra)
Jizo (Ksitigarbha)
Miroku (Maitreya)
Yakushi (Bhaisajyaguru)
Kannon (Avalokitesvara)
Seishi (Mahasthamaprapta)
Amida (Amitabha)
Ashuku (Akshobhya)
Dainichi (Vairocana)
Kokuzo (Akasagarbha)


Minor update: I did some googling on this, it appears that lists of Buddhas, sometimes very long lists of Buddhas, occur throughout the Mahayana cannon.
Courtesy of Brian Gentz 

The list of 13 Buddhas used in Shingon funeral rites is based on the chief Buddhist deities found on two important mandalas used by Shingon practitioners in ritual practices: the Taizokai (Womb World) mandala and the KongoKai (Diamond World) mandala.  It is understood the deities assist the living and the deceased by guiding them into the realm of enlightenment.  Depending on the anniversary date of the deceased, a memorial service associated with the deity is held for the deceased.

Each deity on the list plays an important part at different levels of the two mandalas in assisting the initiate reach enlightenment.   These mandalas are aggregates of additional mandalas from the early Vajrayana tradition.  The mandala principals for the illustrations and placement of the deities roles are found in early Vajrayana texts: the Womb World Mandala is found in the Mahavairocana Tantra, and the Diamond World Mandala is found in the Vajrasekhara Tantra.

List of 13 Buddhas and the dates for the corresponding memorial services:


Fudo (Acala) - The Immovable Radiant King, 7th day 
Shaka (Sakyamuni) - Sage of the Shakya Tribe, 14th day
Monju (Manjushri) - Bodhisattva of Beautiful Splendor, 21st day
Fugen (Samantabhadra) - Bodhisattva of Universal Beauty, 28th day
Jizo (Ksitigarbha), - Bodhisattva Storehouse of the Earth 35th day
Miroku (Maitreya), - The Benevolent Bodhisattva 42nd day
Yakushi (Bhaisajyaguru), - Buddha of the Master of Medicine 49th day
Kannon (Avalokitesvara), - Bodhisattva of Compassion 100th day
Seishi (Mahasthamaprapta), - Bodhisattva Who Proceeds With Vigor 1st anniversary
Amida (Amitabha), - Buddha of Infinite Life and Life 2nd anniversary
Ashuku (Akshobhya), - The Immovable Buddha 6th anniversary
Dainichi (Vairocana), - The Great Buddha of Universal Illumination 12th anniversary
Kokuzo (Akasagarbha), The Storehouse of Space Bodhisattva 32nd anniversary

Garbhadhatu Mandala (Womb World Mandala)




Courtesy of Peehyoro Acala

Vajradhātu Mandala (Diamond World Mandala)



Courtesy of Peehyoro Acala