Scripture | Tradition | Reason
+ + +
Readings are prescribed for each Sunday: a passage from the Old Testament, the Apocrypha, or the Acts of the Apostles; a passage from one of the Psalms; another from either the Epistles or the Book of Revelation; and finally a passage from one of the four Gospels.
The Revised Common Lectionary follows a three-year cycle; the Gospel readings in the first year (Year A) are taken from the Gospel of Matthew, those in the second year (or Year B) from the Gospel of Mark, and in the third year (or Year C) come from the Gospel of Luke.
Portions of the Gospel of John are read throughout Eastertide, and are also used for other liturgical seasons including Advent, Christmastide, and Lent where appropriate.
Year A begins on the first Sunday of Advent in 2010, 2013, 2016, 2019, etc.
Year B begins on the first Sunday of Advent in 2011, 2014, 2017, 2020, etc.
Year C begins on the first Sunday of Advent in 2009, 2012, 2015, 2018, etc.