The second most frequently used book at Mass is the Lectionary. These books contain the Scripture readings proclaimed at all Masses throughout the liturgical year. In the Roman Catholic Church, the Lectionary is organized into four volumes, arranged according to liturgical celebrations:

   +Sundays, Solemnities, and Feasts of the Lord

   +Weekdays (Advent to Easter; Saints, December–June)

   +Weekdays (Ordinary Time; Saints, June–November)

   +Ritual Masses, Votive Masses, and Masses for the Dead

For Sundays, the Roman Catholic Church uses a three-year cycle of readings, labeled Years A, B, and C. Each year emphasizes a particular Gospel evangelist:

   +Year A focuses on Matthew

   +Year B focuses on Mark

   +Year C focuses on Luke

The Gospel of John is proclaimed across all three years, especially during the major seasons of Advent, Christmas, Lent, and Easter. This three-year lectionary cycle was developed following Vatican II and allows for a much broader exposure to Sacred Scripture. Prior to Vatican II, the lectionary followed a single-year cycle, meaning the same readings were heard each year.

Weekday Masses follow a two-year cycle. Year I is used in odd-numbered years, and Year II in even-numbered years. The Gospel readings remain largely the same in both cycles, while the first readings differ. Year I draws more heavily from the Old Testament, especially narrative passages from Genesis, Exodus, Samuel, Kings, and the prophets. Year II places greater emphasis on the epistles of the New Testament.

Together, these Sunday and weekday cycles provide extensive coverage of the Bible and reflect the Vatican II principle “to open up the treasures of the Bible more lavishly.” In practical terms, this reform means that Catholics today hear several times more Scripture proclaimed at Mass than in the centuries immediately preceding the Council. This represents one of the most dynamic changes to the modern Mass. The Roman Catholic Church was the first to implement a modern three-year lectionary cycle, which was later adopted and adapted by many mainline Protestant traditions, including Lutherans, Presbyterians, and Methodists.

The organization of the readings follows a clear internal logic. On Sundays, the first reading is chosen for its thematic connection to the Gospel, which proceeds in semi-continuous order through the assigned Gospel for that year. The responsorial psalm, sung or recited, serves as both a musical and theological response and often reinforces the shared themes of the readings. The second reading is usually not directly related and instead moves sequentially through a New Testament book over several weeks.

At weekday Masses, the first reading and Gospel typically progress sequentially through their respective books and are not intentionally linked thematically. The psalm is selected to respond to the Gospel. Exceptions to these patterns occur on solemnities, feasts, memorials of saints, and during the seasons of Advent, Christmas, Lent, and Easter.

In the Roman Catholic tradition, great significance is placed on both form and words, especially in the celebration of the sacraments. This reverence carries over into the Liturgy of the Word, where Christ is made present through the proclaimed Word of God, read from sacred books at the ambo. For this reason, readings are not proclaimed from tablets or photocopies. Lectionaries are designed with both beauty and function in mind, are blessed and set apart for sacred use, and are treated with the reverence due to the Word of God.