It's unclear whether you are asking why the author seems to approve of Christianity, or why wizards do in-universe, so I have attempted to answer both possible questions.
Given that the author herself identifies as a Christian the presence of Christian holidays is no surprise. I think her own words are the best answer to your question: "I don't take any responsibility for the lunatic fringes of my own religion." So she is dismissive of those groups which consider her books unchristian, but there is no reason for her to be negative about her own religion.
As far as why wizarding culture accepts Christianity, we can reasonably conclude that is is because many wizards in England are in fact Christians. There is no reason to believe otherwise. Just because a group is unpopular with more mainstream Christianity doesn't mean they can't identify as Christian. For example, many gay people in real life identify as Christian regardless of the fact that many branches of Christianity oppose homosexuality. The same phenomenon probably applies to wizardry.
Evidence of this is present in crystallized answer on another question :
That being said, in Harry Potter and the Deathy Hallows, Harry finds
two tombstones with Bible (New Testament) quotes on them. His parents'
reads "The last enemy that shall be destroyed is death" (1 Corinthians
15:26) while Kendra and Ariana Dumbledore's reads "Where your treasure
is, there will your heart be also" (Matthew 6:21)