This article deals with the Charmosyna or “the Festival of Rejoicing" in the light of Greek papyri uncovered from Graeco-Roman Egypt and other textual evidence. It discusses the different associations of the Charmosyna with the Apis of Memphis, the god Osiris, the god Serapis, and the goddess Isis. During the Late Period, the Charmosyna is first confirmed in Herodotus‟ Histories in connection with the birth of a new Apis at Memphis. In the late Hellenistic Period, textual evidence from Kios (Bithynia, Asia Minor) associated the Charmosyna with the goddess Isis, suggesting that the festival was more than a local Egyptian celebration. In the early second century AD, Plutarch associated the Charmosyna with the god Serapis, who was a fusion of Osiris-Apis. In the mid-second century AD, the Festival of Rejoicing is last mentioned in the festival calendar of Soknopaiou Nesos, which gives eight days from the 16th to the 23rd of Phaophi (Julian: 13th-20th of October) for the festival.