As an Episcopal musician I live and breath the Revised Common Lectionary (RCL). The lectionary determines the readings for every Sunday of the year and thus guides the music selected for each service. Because music must be rehearsed weeks in advance with the choirs, it is very helpful to be able to know what the themes will be for any given Sunday at any time.
The notion of appointing particular readings for specific days can be found throughout the Jewish tradition back to the time of Moses. The early Christian church continued this practice and through the various church Councils a thorough system of readings for each day and week of the year was gradually developed by the Catholic Church. Following Vatican 2 in the 1960's the Catholic Church adopted a new revised lectionary which is still in use today and is the basis for a series of several lectionaries that eventually led to the Revised Common Lectionary which is now widely used by most denominational Christian churches other than the Catholics who, of course, still use their own.
The Revised Common Lectionary provides lessons from the Old Testament, Psalms, Epistles and Gospels for each Sunday. It works on a three year cycle with each year focusing on one of the three synoptic
Gospels. We are currently in Year A which is Matthew, Year B is Mark, and Year C is Luke, with the Gospel of John being used more seasonally. The remaining lessons then support the theme of the Gospel. This is then intricately balanced with the themes of the various liturgical seasons and Holy Days to create a system which allows for an orderly progression through the church year which roughly follows the life of Jesus. The church year begins with Advent when we focus on the prophecies of, and preparation for, the birth of Jesus which we then celebrate at Christmas. Twelve days later, at the Epiphany, Jesus is shown forth to the world as represented by the visit of the Wise Men. In the Sundays following, the lessons focus on the teachings and ministry of Jesus beginning with his baptism, healings and miracles. During Lent we prepare for Jesus' crucifixion and death and then celebrate his Resurrection at Easter. The lectionary then uses lessons of Jesus' post Easter appearances, his ascension into heaven, and ends with the day of Pentecost when the Holy Spirit is revealed to the disciples. After Pentecost the lectionary enters Common Time until the next Advent.
For those who come from non-liturgical backgrounds, the lectionary can seem rigid and not allowing for spontaneity. However, I think it offers many tremendous advantages. In addition to being a very useful planning tool, the lectionary, and the liturgy it supports, creates a systematic framework for our Christian journey. One could think of it as a study guide for our spiritual growth which insures we will cover all the pertinent topics in an order that makes sense and is not subject to the whims of others.
Finally, the lectionary is a wonderful instrument of unity. Both the Revised Common Lectionary and the Catholic lectionary, which is nearly identical, are used in churches across the globe. On any given Sunday we can know that our fellow Christians all around the world are reading the same lessons and experiencing the same stories as us. We are all on the same journey together. Let us rejoice and be glad!