In chapter 4 where Paul shows concern that the Galatians were "observing days and months and seasons and years!" which correspond to Jewish Sabbath days, festivals and feast days.
And also where Paul uses the metaphor of the Slave Woman and the Free Woman to correspond to the Old Covenant [the Law] and the New Covenant [Life in Christ], we look at those distinctions and what they mean for us today.
Keep in mind that Paul ends this epistle by saying: "And as for all who walk by this rule [the New Covenant reality found in Christ], peace and mercy be upon them, and upon the Israel of God." ch. 6 v. 16
So, the "Israel of God" are those who are in Christ, not those who keep the Law and look to the keeping of feasts, festivals and sabbaths.