Where does the First reading in a Catholic mass come from?

Answer:

In the Catholic Church, there are two different cycles of readings, one for Sunday Masses (allowing the Church to read nearly the entire Bible in three years on Sundays) and one for weekday Masses (which covers the same amount of the Bible in two years). It is important to remember that Sunday Masses have three readings while weekday Masses have two.

During Sunday Masses, the first reading usually comes from the Old Testament, with rare exceptions including the Sundays of the Easter Season, when the first readings come from the Book of Acts. Also, during Advent, the first reading comes from the book of Revelation on Sundays. During weekday Masses, the first reading comes from either the Old or New Testament.