The god Serapis is a syncretic deity comprised of Wesir/Osiris and Apis; he also gained attributes from Greek deities such as Hades and Zeus. He became the consort of Isis within the Ptolemaic and Roman periods. To read more about Serapis here is the Wikipedia entry: Serapis.
Festivals to Serapis were called Serapia or Serapieia. During these festivals, Serapis (along with Isis in some cases) was honored with libations, sacrifices and offerings. There is mention of one festival in Alexandria including a procession with torches in his honor. I included some Osirian festivals in the list since the Perpillou-Thomas’s book did the same.
The Egyptian and Classical dates for these festivals are listed below. The Egyptian ones have the number of the days corresponding to the Egyptian month on the calendar and the Classical ones have the Julian dates associated with them. l
I also began my New Year in August. If one begins the New Year in a later month, then Pachons isn’t going to be in April.
Some dates for these festivals are:
17 to 20-Festival of Aset, with the Beautiful Throne (Isis Nepherses)
26-Festival of Serapis/Serapia
24 to 29-Mysteries of Wesir
30-Festival of Serapis/Serapia
7-Festival of Serapis/Serapia
10-Festival of Serapis/Serapia
mid-Month of April-Festival of Serapis/Serapia
mid-Month of Pachons-Festival of Serapis/Serapia
25 of April-Festival of Serapis/Serapia
Extra Day 1-Birthday of Wesir/Osiris
Bricault, Laurent. Les Cultes Isiaques Dans Le Monde Greco-romain (La Roue a Livres / Documents Book 66). Les Belles Lettres, 2013. pages 371-375.
Capron, Laurent. “Déclarations fiscales du Temple de Soknopaiou Nêsos: éléments nouveaux,” in Zeitschrift für Papyrologie und Epigraphik. Bd. 165, Dr. Rudolf Habelt GmbH, Bonn (Germany). (2008), pp. 133-160.
Perpillou-Thomas, Francoise. Fêtes d’Egypte ptolémaïque et romaine, d’après la documentation papyrologique grecque. (Studia Hellenistica Series 31). Peeters Publishers, 1993. pages 129-136.