As we read in Leviticus today we see that God appoints a routine of festivals for the people of Israel. Of course, they are centered around the Sabbath worship which he has instituted. Yet on certain Sabbaths there are special emphases which the Lord has commanded. Each one has a special focus - trumpets to show God's self-proclamation, a Day of Atonement for forgiveness, a time in booths to remember the pilgrim nature of God's people.
It's a good thing to remember what God has done in the past. It's a good thing to have special days when we focus on important events. In past years I spent time in several different church congregations which took little or no notice of the historic Church calendar. One of them went so far as to have a special evening service near Christmas but otherwise made no official mention of even Christmas or Easter. While I can understand the convictions that might lead toward this - a desire to emphasize the systematic preaching and teaching of the Scripture in its context through extended passages of the Bible (why let Easter interrupt your series on Isaiah?), I don't think this is well advised. God has worked in very distinctive ways at different times in history. There's a sort of rhythm in the Church year. It provides a framework on which to hang many aspects of our Christian belief, simply through noting the seasons believers throughout history have acknowledged.
Our Lord has made the times and the seasons. He has appointed them for reasons. And though the Christian calendar is not specifically commanded by the Lord, as were these feasts in Leviticus, we can use that calendar to the benefit of God's people, reaching out to our world, pointing them to what our Lord Jesus Christ has done.