On the Western Christian calendar, the First Sunday in Advent begins a new church year and the season of preparation for Christmas. When Christmas entered the Latin calendar during the fourth century, Advent, which comes from the Latin word "coming," became a time to emphasize the theme of Christ's coming into the world.
We see this theme of Christ's Coming in the Gospel selection from Matthew 21:1-13. At this time of the year, the account is not so much about the historical details of Christ in Jerusalem as it is a reminder of the varied ways that Jesus Christ comes into human lives. Christ the Messianic King has already come into the world long ago, He keeps coming to us through the work of the Holy Spirit in Word, Sacrament, and prayer, and Christ will come again to conclude earthly history on the last day.