What is the X in X-mas?
late 14c., "assembly of persons," from Old French convocation and directly from Latin convocationem (nominative convocatio), noun of action from past participle stem of convocare "to call together," from com- "together" (see com-) + vocare "to call," from vox "voice" (see voice (n.)). Related: Convocational.
a meeting of a religious character as distinguished from congregation, which was more general, dealing with political and legal matters. Hence it is called an "holy convocation." Such convocations were the Sabbaths (Lev. 23:2, 3), the Passover (Ex. 12:16; Lev. 23:7, 8; Num. 28:25), Pentecost (Lev. 23:21), the feast of Trumpets (Lev. 23:24; Num. 29:1), the feast of Weeks (Num. 28:26), and the feast of Tabernacles (Lev. 23:35, 36). The great fast, the annual day of atonement, was "the holy convocation" (Lev. 23:27; Num. 29:7).