As per usual, the context explains what St. Paul was talking about. The verses immediately preceding 10-11 tell us what he was warning the Galatians against:
 Formerly, when you did not know God, you were in bondage to beings that by nature are no gods;
 but now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and beggarly elemental spirits, whose slaves you want to be once more?
The Galatians had been pagans who worshiped pagan gods according to the customs of those gods. They were continuing to do so even though they no longer worshiped those gods–probably because everyone else was and they had celebrated that way all their lives.
The passage has absolutely nothing to do with the seasons and holy days of the Church, which hadn’t even been forulated yet. How could the Galatians have been guilty of keeping the Feast of the Nativity, for example, when it wasn’t yet established? And no, it doesn’t mean that once the Church had established them that it was wrong for the Church to do so. And most certainly not on the basis of these two verses (10-11), which have nothing whatsoever to do with it.
This type of interpretation is called “fishing” because a anti-Catholic goes fishing in the Bible for verses that appear to support his position but only if he first tells you that “the Church has seasons and years and holy days so these verses must be talking about them and saying that it’s wrong.” Of course, he has to ignore the context completely to come to that conclusion and he has no historical basis to back him up, but never mind, he just knows it refers to that “evil” Catholic Church, so that’s what it has to mean. :rolleyes: