If you attend Mass regularly you'll notice certain books hold prominence of place in the gathering. These over-large and often decorated volumes contain Bible passages appointed for public reading by trained readers (lectors) and are called lectionaries.

Each lectionary organizes readings according to the feasts and seasons of the church year. In this way we hear about the birth of Jesus at Christmas, the Passion during Holy Week, the ministry of Jesus during other times of the year, and so on. The Sunday lectionary contains three years' worth of readings:

—Cycle A follows Matthew's gospel with Old Testament passages chosen to parallel its themes.

—Cycle B is organized around Mark's gospel—although Mark is so short that John's gospel supplements the year.

—Cycle C coordinates Luke's gospel with Old Testament readings.

(The Gospel of John isn't slighted; it's used in all three years for special feasts when thematically appropriate.)

In between the Old Testament and gospel readings on Sundays, an additional New Testament passage is selected from a letter of Saint Paul or another apostle and read continuously across the Sundays until it's finished. During the Easter season the Old Testament reading is replaced by a passage from the Acts of the Apostles or the Book of Revelation.

There's also a daily lectionary that runs in a two-year cycle (Years I and II) pairing gospel passages with continuous readings from Old or New Testament books. Saints' days have their own appropriately chosen optional readings, and an additional lectionary has passages suitable for baptisms, weddings, funerals, and other occasions.

Why do we have lectionaries? For one thing, they provide breadth. Catholics can hear a fairly broad amount of scripture in a few years' time. Not every Bible verse is covered by the lectionary, but a surprisingly comprehensive reading can be achieved by the daily Mass-goer.

Another practical reason for lectionaries is that they save time: The preacher doesn't have to scramble looking for passages on forgiveness for every Rite of Reconciliation, for example. Finally, it keeps preachers honest: They can't default to their favorite themes but must treat scripture in its fullness.

Lectionaries have existed in one form or another since Christianity's Jewish roots in the synagogue. These tools have proven the test of time.

Scripture
Nehemiah 8:1-12; Psalm 119; Luke 4:16-21; Hebrews 4:12; James 1:19-21

Website
The New American Bible organized by daily lectionary readings

Books
Journeying with Mark (also available for Matthew and Luke) by Jennifer Christ (Paulist Press, 2005)
God's Word Is Alive: Reflections on the Lectionary Readings by Alice Camille (ACTA Publications, 2007)