Galatians 2:4 warns of false brothers who spy out freedom to bring slavery — the same slavery the Galatians are returning to by observing days.

Leviticus 23:1-44 lists the appointed feasts that Paul says believers should not feel obligated to observe.

Romans 14:5 permits each person to decide about observing days — a contrast to Paul's rebuke in Galatians of such observance as a return to bondage.

Colossians 2:16 forbids letting others judge you regarding festivals, new moons, or Sabbaths — the same list Paul rebukes the Galatians for practicing.

Colossians 2:17 explains that these observances are only a shadow, with the reality in Christ — the theological reason the Galatians should not observe them.

Romans 14:6 says observing a day can be done in honor of the Lord — a different perspective from Paul's warning in Galatians about returning to weak elements.

Numbers 28:11 commands new moon offerings — the OT law behind the 'months' the Galatians were observing.