Ye observe days, and months, and times, and years.
Note 3 at Ga 4:10: Paul was rebuking the Galatians for trying to incorporate observance of the Old Testament Law into the Gospel that he had brought to them (see note 8 at Ga 1:7). Here, he named some of the things they were proclaiming had to be done to receive right standing with God.
The observance of days refers to keeping the Sabbath (see note 1 at Joh 5:16). Observing months and times is a reference to the new moons (Nu 10:10, 1Ch 23:31, and Ps 81:3) and feasts (i.e., Passover, Firstfruits, etc.). Observing years refers to the Sabbath year and the Year of Jubilee of Le 25. Paul made it very clear that the keeping of these rituals is not necessary for salvation. Those who preach that it is are dangerously close to having their faith in Christ voided (Ga 2:21), as Paul described in the next verse.