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The Romans took this feast originally from the Greeks, who called it ΑΝΑΒΑΣΙΣ, q.d. Ascensus: the eve of that day they spent in tears and lamentations, and denominated it ΚΑΤΑΒΑΣΙΣ, Descensus. Afterwards, the Greeks took the name ΙΛΑΡΙΑ, from the Romans, as appears from Photius's Bibliotheca, in his codex of the life of the philosopher Isidore of Alexandria.
The term seems originally to have been a name which was given to any day or season of rejoicing. The hilaria were, therefore, according to Maximus Monachus either private or public. Among the former, he thinks it the day on which a person married, and on which a son was born; among the latter, those days of public rejoicings appointed by a new emperor. Such days were devoted to general rejoicings and public sacrifices, and no one was allowed to show any symptoms of grief or sorrow.
But the Romans also celebrated hilaria, as a feria stativa, on March 25, the eighth day before the Kalends of April, in honor of Cybele, the mother of the gods; and it is probably to distinguish these hilaria from those mentioned above, that the Augustan History  calls them Hilaria Matris Deûm. 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 template 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.
The full festival can be tentatively reconstructed (with the days of the festival literally translated) as follows:
Herodian details an assassination plot by Maternus against Emperor Commodus that was to occur on the hilaria. Maternus planned to disguise himself and his followers as members of the Praetorian Guard, and proceed among the true members of the Guard, until they were close enough to kill Commodus. However, one of Maternus's followers revealed the plot ahead of time, betraying him because, according to Herodian, his men "preferred a legitimate emperor to a robber tyrant". On the day of hilaria, he was beheaded and his followers punished. The public celebrated the emperor's safety, and Commodus sacrificed to Cybele for protecting him from harm.