These revered Buddhas include the 28 Buddhas described in Chapter 27 of the Buddhavamsa, plus Maitreya Bodhisattva, the future (and 29th) Buddha. The Buddhavamsa is a text which describes the life of Gautama Buddha and the twenty-seven Buddhas who preceded him. The Buddhavamsa is part of the Khuddaka Nikaya, which in turn is part of the Sutta Pitaka. The Sutta Pitaka is one of three main sections of the Pali Canon of Theravada Buddhism.
The 28 Buddhas are said to have attained enlightenment from the time Gautama Buddha received his first Niyatha Vivarana (permission to be the next enlightened one) from Dīpankara Buddha.
The 28 Buddhas are not the only Buddhas believed to have existed. Indeed, Gautama Buddha preached that innumerable Buddhas have lived in past kalpas.
In most Theravada countries it is the custom for Buddhists to hold elaborate festivals to honor 28 Buddhas. For example, in various parts of Myanmar, festivals are held to pay homage to 28 Buddhas, especially in the fair weather season. The twenty eight Buddhas are said to have attained enlightenment from the time Gotama Buddha attained his first definite prophecy (assurance that one will definitely become a Buddha one day) from Dipankara Buddha.
The names of the 28 Buddhas
27. Kassapa Buddha
28. Gotama Buddha