Christmas is unique among Feasts in that there are 4 Proper Masses for it, with their own antiphons, readings, and prayers: the Vigil ("Advent Mass"), Mass During the Night ("Angels' Mass"), Mass at Dawn ("Shepherds' Mass"), and Mass During the Day ("Divine Word/Kings' Mass").
At the Vigil we expect the realization of our Advent Hope as the Introit says, "Today you will know that the Lord is coming to save us; and tomorrow you will see his glory". The Offertory proclaims, "the King of Glory shall make his entry". It's still Advental, looking to the day
At Midnight Mass we finally see the Savior's glory, "today I have begotten you" says the Introit. The Gospel proclaims the Nativity then tells of the Angels who beckon all to Bethlehem. Before Mass the great Christmas Proclamation definitively announces the Feast.
At Dawn, the Introit's verse (Ps 93) sings, "The LORD is king, with majesty enrobed. The LORD has robed himself with might; he has girded himself with power." The Gospel tells of the shepherds who heeded the angels and traveled into Bethlehem "glorifying and praising God".
Finally, the Day Mass proclaims the realized hope of Advent. The Introit sings, "Unto us a child is born, unto us a son is given", and its verse (Ps 98) continues, "O sing a new song to the LORD, for he has worked wonders. His right hand and his holy arm have brought salvation."
