Christian derive the Christmas Eve tradition from the Jewish tradition of the holy days beginning at sundown of the previous day. Christmas is understood as beginning with sundown on the 24th of December and ending at sundown on the 25th and then immediately followed by the Feast of St. Stephen. The Church generally has four different Masses for Christmas: the Vigil Mass, the Mass at Midnight, the Mass at Dawn and the Mass during the Day, although not all Masses are necessarily celebrated in the same Church. The Mass at Midnight has long been the most popular Christmas Mass but since the Vatican II reforms, the Christmas Vigil Mass has become more popular for families.
Many families have traditions and rituals that are now part of the Christmas Eve celebrations. Generally there is a shared meal, followed by the celebration of the Eucharist, then shared time with families that goes past midnight and can also include the opening of gifts, eating certain foods, visitation of St. Nicholas, turning all lights off except for the lights of the tree, etc.