Advent / Christmas
Ordinary Time 1 (2nd Sunday - 8th Sunday)
Lent / Easter
Ordinary Time 2 (Trinity Sunday - 22nd Sunday)
Ordinary Time 3 (23rd Sunday - 34th Sunday)
Per Vatican II, a Lectionary, was developed with three yearly cycles of readings for Sunday and Holy Days to give the faithful a chance to hear as much of the Scriptures as possible.
On weekdays, every Mass has two readings. The first is from the Old Testament or from a letter. During the Easter Season the reading is from the Acts of the Apostles or the book of Revelation. The second reading is from a Gospel. There is a one year cycle of readings for the Gospels, and it is repeated each year. The first reading has a two-year cycle. A different reading is read on alternate years. The first is read on odd numbered years and the second on even numbered.
There are special readings for the feasts of our Lord and his mother Mary, for important feasts of the saints, and for special needs and occasions.
YEAR A: The Gospel of Matthew is read
YEAR B: The Gospel of Mark is read
YEAR C: The Gospel of Luke is read
The Gospel of John focuses on the risen life of Christ, so it is read primarily during the Lent/Easter season and secondarily during the Advent/Christmas season.