Christian liturgy is closely tied to the Christian calendar. The Urban Ministry Institute, which produces a lovely calendar each year, notes:
Since our earliest records, the Church has made it a point to remember and celebrate the events of the life of Jesus of Nazareth, the Christ. The ancient Church centered its worship and witness on the person and work of Jesus Christ. A cursory outline of Church history reveals its deep commitment to remember and re-present the major events of Jesus’ life in real time. Largely as an act of worship and instruction, the Church Year was the early Church’s response to the secular events and calendar that sought to structure and give meaning to society’s course of life.
The Christian Year (also known as the Church Year) observances, feasts, and services allowed them to think “Christianly” about their days and weeks and months, and put all things in the perspective of the Christ event.
Used at least in part by virtually all traditions of the Church (including Catholic, Orthodox, Anglican, and Protestant traditions), the Christian Church year highlights and follows the prophecy, manifestation, and ministry of Jesus. The unfolding events on the calendar becomes an opportunity for worshipers to hear the prophets herald his coming, to kneel at the manger, to worship Christ with the magi, and to hear his teaching to the multitudes. Through these events we see our Lord triumphantly come to Jerusalem, stand accused in a sham trial before his foes, be crucified with thieves on Golgotha, and rise from the dead on the third day! From his ascension to the coming of the Holy Spirit, from his exaltation to the mission of his Church in the world, the Church Year reminds us of the that Story which in fact is the Greatest Story Ever Told– the hope of salvation in Christ for the world.
The purpose of the Church Year, then, from ancient times to the present, has been to remember, reenact, and to be transformed by the major events of Jesus’ life. In the course of our everyday lives, we redraw and relive the way of Jesus in real time, tracing his birth, death, resurrection, ascension, session and return through the course of the year.
As Gilbert Le Blanc answered, the Revised Common Lectionary is currently used by a wide variety of Christian traditions. But it is based on the practice of Bible readings that go all the way back to the 1st century church and the practice of regular readings (miqra) in the Jewish synagogue. Rev. Alexander Ring detailed the history of the lectionary and the church calendar in this article.