Days run from midnight to midnight, right? Sometimes. For Sundays and things like Christmas and Easter, the day starts the evening of the day before, on the “eve.” Which means the evening mass on Christmas Eve is a mass (the first of four) of Christmas Day.
Christmas Eve pulls together all of Advent. Especially with the first two readings. The Old Testament reading (Isaiah) rejoices as all the prophecies are brought to fulfillment. The Epistle (Acts) recalls God’s promise to David.
And then the Gospel (Matthew) bogs down in the genealogy of Jesus. Why? To show how God's promise to David is being fulfilled in Jesus.
There’s a saying that “Behind every great man there’s a great woman.” The Christmas Eve Gospel turns that upside down, showing us the great man behind the greater woman. And the silent “yes” to God behind the greatest “yes” to God.