The prophetically-accurate Sibylline books had revealed to the Romans that with the foreign Magna Mater they would be able to defeat Carthage in the war Rome was struggling to win against Hannibal. So, on the day in 203 B.C. that Romans first carried the stone simulacrum of the Phrygian goddess Cybele to Rome, they feted her with a procession and games.
After completing Cybele's temple, in 191 B.C. on the tenth of April, dedicatory games were held that were described as "scaenici" and referred to as the megalesia by Livy [Lily Ross Taylor]. Afterwards, Rome celebrated the Megalesia each year, with feasting, extravagance, and performances before her temple. Taylor says the first and last days of the games were festivals in honor of the goddess. The magistrate in charge put on Cybele's games -- dramatic presentations (no racetracks, gladiators or beast fights), including four of Terence's comedies and Plautus' Pseudolus and possibly Trinummus, as well [Taylor]. [See the Golden Age of Latin Drama.] Scheid says it was the praetor who celebrated the games, and probably the praetor urbanus, basing his opinion on Dion. Hal. Ant. Rom. 52.19.
Some Technicalities
In "The Dates of the Megalesia," Transactions and Proceedings of the American Philological Association, 1930, George Depue Hadzsits points out that the 3rd and 11th of April may have been opening (and presumably, closing) days for the events. He also says that not all the games during this period were for the Megalesia; that some were for other concurrent anniversaries. Taylor says the dates of April 4-10 were those of the Augustan calendar and speculates that between those end points were six days of games.
"Graeco Ritu: A Typically Roman Way of Honoring the Gods"
John Scheid
Harvard Studies in Classical Philology, Vol. 97, Greece in Rome: Influence, Integration,Resistance (1995), pp. 15-31
"The Opportunities for Dramatic Performances in the Time of Plautus and Terence:
Lily Ross Taylor
Transactions and Proceedings of the American Philological Association, Vol. 68 (1937), pp. 284-304
