Paul wrote to the Galatians, Gal 4:10 You are observing days, months, seasons, and years. 11 I am afraid for you! I don’t want my work for you to have been wasted!
The Galatians had lapsed back into celebrating the feasts and days of Baal, which was derived from the cult of Nimrod through the religion established by his mother and widow, Semiramis!
Ah, I wouldn't go so far as to claim this.
I think it is perfectly clear that in the area of Galatia, as in other areas, Judaizing quasi-christfollowers were going out to try to get local assemblies to reincorporate circumcisions, Seders, hand-washing, and other little rituals back into practice thus clinging to the Jewish culture whilst claiming the New Covenant--sort of like "Messianic" congregations are attempting today. I believe that kind of "gospel" was really what Paul was condemning.
I will certainly agree that, on the other hand, the Gentile wannabe "evangelists" were influencing their rude adherents to also just "Christianize" their pagan feasts. And that slant would also be an illegitimate non-gospel approach to obtaining willing "converts."
This gets followers of a religion to focus on demonstrating to themselves and others that it is the religious observances that one does rather than placing the emphasis on what you are that identifies a "Christian."
This link explains fairly well, without much sectarian bias.
http://www.rayfowler.org/writings/articles/determining-the-dates-for-easter-and-passover/