A lectionary is a book of collections of scriptures to be read during a particular day for Catholic churches. A lectionary for mass is a book of scriptures meant to be read during mass, which is the sacrament service for the Catholic church.
The practice of having books of scriptures selected to read upon a certain day dates back to the time of Moses. Lectionaries are set up logically, with picked scriptures relating in a specific order to the day or event in which they are being read.
Lectionaries are typically set up in 1- and 3-year cycles, with the books being changed after the cycles are complete. The original practice of using such materials was adopted from Judaism, which is another religion that still uses the lectionary system.
Lectionaries are used during Sundays, feast days and for the celebration of saints. Lectionaries are typically set up to include a reading from the Old Testament, then a section to be sung, followed by a reading from the New Testament and lastly a reading from the Gospels. Some of the 3-year lectionaries are set up to be read one book per year as follows: the gospel of Matthew, the gospel of Mark and the gospel of Luke.