The early church celebrated all of God's Holy Days because they teach about Y'shua (Jesus).
During the following years, the church grew and became predominantly Gentile. Many wanted to keep their pagan customs and did not want to appear to be Jewish, so the celebration of God's Holy Days fell by the wayside. Thus began the great apostasy and the Dark Ages.
What do we do now? Study God's Word - Read about the Holy Days - Attend a Passover Demonstration -- learn about how it teaches of Y'shua -- There are many books written by believers to show how we can honor God by celebrating His Holy Days. This is a joyful thing we should do because we love the Lord - it is not a legalistic requirement. Our chief reason for being created is to serve God and enjoy Him. Celebrating the Holy Days helps us know Him more intimately and understand just what He did.
I have taught about the Holy Days for years, and I am still learning new things about the Lord. It is a source of great blessing! Have fun!