The occasion to be joyful about is Christmas. The word Christmas is derived from “Christ’s Mass” which has its roots in the Middle English “Christemasse” and the Old English “Cristes mæsse.”
Christ, of course, refers to Jesus Christ. The “mass” in this case comes from the English meaning and tradition of a religious feast in honor of a specific person. Christmas Day is the feast day of Christ. Christmas also means the larger season around Christmas Day or the 12 days of Christmastide.
Christmas day, December 25th, is the celebration of Jesus’ birth. The announcement of the Angel to the Shepherds in the Gospel of Luke clarifies the sentiment behind “Merry Christmas“: “…the angel said to the Jews, “Do not be afraid. I bring you good news of great joy that will be for all the people. Today in the town of David a Savior has been born to you; he is Messiah the Master.”