December 24 falls on a Sunday this year. Note there are two obligations – for the 4th Sunday of Advent and for the Solemnity of Christmas. The obligation for the 4th Sunday of Advent is fulfilled by an anticipatory Mass on Saturday, December 23 , or on Sunday morning. The obligation for Christmas (a Holy Day of Obligation) is fulfilled by Mass in the late afternoon/evening of December 24, or on December 25.