Hilaria, in Roman religion, day of merriment and rejoicing in the Cybele-Attis cult and in the Isis-Osiris cult, March 25 and November 3, respectively. It was one of several days in the festival of Cybele that honoured Attis, her son and lover: March 15, his finding by Cybele among the reeds on the bank of the River Gallus; March 22, his self-mutilation; March 24, fasting and mourning at his death; and March 25, the Hilaria, rejoicing at his resurrection. Some of the activities on the Hilaria resembled those associated with April Fools’ Day. November 3, the Hilaria of the Isis-Osiris cult, marked the resurrection of Osiris, husband of Isis.