Note 3 at Gal. 4:10: Paul is 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 Gal. 1:7, p. 1054). Here he names some of the things they were proclaiming had to be done to receive right standing with God.

The observance of days is referring to keeping the Sabbath (see note 1 at Jn. 5:16, p. 154). Observing months and times is a reference to the new moons (Num. 10:10; 1 Ch. 23:31; Ps. 81:3) and feasts (i.e. Passover, Firstfruits, etc.). Observing years is referring to the Sabbath year and the Year of Jubilee of Leviticus 25. Paul is making it very clear that the keeping of these rituals is not necessary for salvation. Anyone who preaches that it is, is dangerously close to having his faith in Christ voided (Gal. 2:21), as Paul describes in the next verse.