Every church that recognizes Easter and Christmas acknowledges a liturgical calendar of sorts. That at least places these two events on their calendars. How would a church justify such acknowledgment?
The primary argument against a liturgical calendar is that scripture doesn’t mention it. From that standpoint, I would argue that observance of a liturgical calendar is not an element of worship for the church. Scripture must regulate the church in all its elements of worship. So I’m saying it is not an element of worship.
All of the emphasis and subject matter of a liturgical calendar, however, is scriptural. From the Bible, we read that Jesus was conceived, was born, was tempted by Satan, was baptized, died, was buried, rose again, ascended into heaven, and is coming back. All of these are in the Bible. Teaching these aspects of Jesus’ life and work is an element of worship.
We don’t have to celebrate Jesus' birth in December or his death and resurrection in March or April. Out of a desire to acknowledge and exalt these particular features of Jesus’ life and works, true believers before the beginning of Roman Catholicism started celebrating these highlights of Jesus. That makes this calendar a circumstance for worship, provoking an orderly acknowledgment of these blessed events.
Jesus was born. God’s Word talks much about His birth and its consequences. Jesus rose again. God’s Word, again, talks much about His resurrection. The problem with any tradition of a church is it becomes mere ritual, like a vain repetition. Let’s be sure not to do that and put our hearts into the worship of Jesus.