On Sundays each Gospel reading has a distinctive theme: the Lord’s coming at the end of time (First Sunday of Advent), John the Baptist (Second and Third Sundays), and the events that prepared immediately for the Lord’s birth (Fourth Sunday).
The Old Testament readings are prophecies about the Messiah and the Messianic age, especially from the Book of Isaiah.
The readings from the Apostle contain exhortations and proclamations, in keeping with the different themes of Advent.2
On weekdays two series of readings are used. The first is from Monday of the First Week of Advent until 16 December, the second from 17 to 24 December.
Readings from the Book of Isaiah, other Old Testament messianic prophecies, readings from the Gospel concerning John the Baptist, and the events that immediately prepared for the Lord’s birth as presented in the Gospels of Matthew (chapter 1) and Luke (chapter 1) are used on the weekdays of Advent. 3
2 Lectionary for Mass: Introduction (1981, 1998), 93.
3 Lectionary for Mass: Introduction (1981, 1998), 94.