What Does Paul Mean in Galatians 4:10?

You are observing days, months, seasons and years. (Galatians 4:10)

This is a reference to ritual observances of the Old Testament. In early Christianity there were groups that insisted that all converts had to observe Mosaic law. The Apostles frequently clashed with these groups and it was the subject of the Council of Jerusalem (Acts 15). What St. Paul is referencing here is the Old Testament observances of sabbaths, Yom Kippur, new moon, sabbatical years, etc.