The Hilaria

In Ancient Roman religious tradition, the hilaria (Greek: ἱλάρια; Latin: hilaris, "hilarious") were festivals celebrated on the vernal equinox to honor Cybele, the mother of the Gods. The Romans took this feast originally from the Greeks, who called it ΑΝΑΒΑΣΙΣ, (Ascensus)

The day of its celebration was the first after the vernal equinox, or the first day of the year which was longer than the night. The winter with its gloom had died, and the first day of a better season was spent in rejoicings.

The manner of its celebration during the time of the republic is unknown, except that Valerius Maximus mentions games in honour of the mother of the gods. Respecting its celebration at the time of the empire, we learn from Herodian that, among other things, there was a solemn procession, in which the statue of the goddess was carried, and before this statue were carried the most costly specimens of plate and works of art belonging either to wealthy Romans or to the emperors themselves.

All kinds of games and amusements were allowed on this day; masquerades were the most prominent among them, and everyone might, in his disguise, imitate whomsoever he liked, and even magistrates.

Source: Wikipedia