Some of you have asked this question several times, so I'm going to attempt giving you an answer. It's not my answer, mind you. It's Wikipedia's response to the question!
In all antiquity the Jews first read Scriptural selections randomly, but by the Medieval era had standardized a schedule of scripture readings to be read in the synagogue. A sequential selection was read from the Torah, followed by the "haftarah" – a selection from the prophetic books or historical narratives (e.g. "Judges," "Kings," etc.). Jesus probably read a providentially "random" reading when he read from Isaiah 61:1-2, as recorded in Luke 4:16-21, when he inaugurated his public ministry. The early Christians adopted the Jewish custom of reading extracts from the Old Testament on the sabbath. They soon added extracts from the writings of the Apostles and Evangelists.
Both Hebrew and Christian lectionaries developed over the centuries. Typically, a lectionary will go through the scriptures in a logical pattern, and also include selections which were chosen by the religious community for their appropriateness to particular occasions.
The use of pre-assigned, scheduled readings from the scriptures can be traced back to the early church. Not all of the Christian Church used the same lectionary, and throughout history, many varying lectionaries have been used in different parts of the Christian world. Until the Second Vatican Council, most Western Christians (Catholics, Old Catholics, Anglicans, Lutherans, and those Methodists who employed the lectionary of Wesley) used a lectionary that repeated on a one-year basis. This annual lectionary provided readings for Sundays and, in those Churches that celebrated the festivals of saints, feast-day readings. The Eastern Orthodox Church and many of the Oriental Churches continue to use an annual lectionary. Within Lutheranism there remains an active minority of pastors and congregations who use the old one-year lectionary, often referred to as the Historic Lectionary.
Catholic Mass Lectionary and the Revised Common Lectionary
After the Second Vatican Council of 1962–1965, the Holy See, even before producing an actual lectionary (in Latin), promulgated the Ordo Lectionum Missae (Order of the Readings for Mass), giving indications of the revised structure and the references to the passages chosen for inclusion in the new official lectionary of the Roman Rite of Mass. It introduced an arrangement by which the readings on Sundays and on some principal feasts recur in a 3-year cycle, with four passages from Scripture (including one from the Psalms) being used in each celebration, while on weekdays only three passages (again including one from the Psalms) are used, with the first reading and the psalm recurring in a 2-year cycle, while the Gospel reading recurs after a single year. This revised Mass Lectionary, covering much more of the Bible than the readings in the Tridentine Roman Missal, which recurred after a single year, has been translated into the many languages in which the Roman Rite Mass is now celebrated, incorporating existing or specially prepared translations of the Bible and with readings for national celebrations added either as an appendix or, in some cases, incorporated into the main part of the lectionary.
The Roman Catholic Mass Lectionary is the basis on which many Protestant lectionaries have been based, most notably the Revised Common Lectionary (RCL) and its derivatives, as organized by the Consultation on Common Texts (CCT) organization located in Nashville, Tennessee. Like the Mass Lectionary, they generally organize the readings for worship services on Sundays in a 3-year cycle, with four elements on each Sunday, and three elements during daily Mass:
3 year cycle
The lectionaries (both Catholic and RCL versions) are organized into three-year cycles of readings. The years are designated A, B, or C. Each yearly cycle begins on the first Sunday of Advent (the last Sunday of November or first Sunday of December). Year B follows year A, year C follows year B, then back again to A. We are currently (November 28, 2010) in Year A (The Gospel of St. Matthew.).
To access the weekly lectionary texts please use the following link: