Scriptures Read at Mass: Whether it is at Daily Mass or Mass on the weekend, the scripture reading is drawn from a liturgical three-year cycle. - Why?
Well, 40 years ago, there was merely one set of readings for Sunday: two readings only, an Epistle, and a Gospel. Each Sunday of the year had two readings, and they never varied from year to year.
Then, in the late 1960s, we received an enriched book of readings (called a lectionary). It's purpose was to provide readings over 3 years, Year A, Year B, and Year C.
The years are also known by the Gospel reading selections for Ordinary Time – Matthew in Year A, Mark in Year B, Luke in Year C. The gospel of John is read during the Easter season in all three years.
Why the change?:
In the late 1960s, the persons who developed these Years (or cycles) of readings had come to realized that in Jesus' time, it took three years to read the Law and the Prophets completely in synagogue worship. With the renewed liturgy, our worship has embraced this same rhythm that Jesus experienced in public prayer.
Similarly, the Order of Readings for Daily Mass lasts only over two liturgical years. Year I corresponds to odd numbered years; Year II corresponds to even numbered years.
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