Since the reforms of the Roman Catholic Mass following Vatican II, Scripture readings for the Sunday liturgy have gone from two readings to three, with the first usually coming from the Old Testament, the second from the New Testament after the Gospels and the third from the Gospels themselves.
The readings of the Roman Catholic Mass follow a three-year cycle consisting of Year A, Year B and Year C. Year A contains a lot of material from the Gospel of St. Matthew, Year B of St. Mark and Year C of St. Luke. The Gospel of St. John is interspersed in these three years. Gospel readings often correspond to the liturgical calendar. Since the church's liturgical year does not coincide with the beginning of the civil calendar, the cycle of readings changes every year on the first Sunday of Advent, which is the beginning of the church's liturgical year.
The church arranges the first and second readings so that they are, if possible, closely related to the Gospel reading, which the church considers the most important of the three readings. Even the Old Testament selections are chosen with special care to highlight their connection to Christ and the New Covenant.