If God does not require a particular behavior, it is wrong to teach it as required. In Judaism, special days were important. But Paul took a rather indifferent attitude to the concept of special days. That is because something significant had happened to change the basis of our relationship with God. Because of that, the old covenant laws came to an end.
Days are no longer a matter for judging behavior.
The Galatians had been pagans before they were saved by faith in Christ. Verse 3, first part. The letter to the Hebrews was written to Jewish believers who were still participating in the customs of Judaism. The letter explains that the old covenant is obsolete and its regulations have been set aside. Throughout the letter, the Jewish believers are reminded that Jesus is much, much better than anything the old covenant had.