A) Yeshu was not born on the 25th. This is the day that the church picked to commemorate his “birth.” In reality since the character named “Yeshu” in the new testament probably never existed, they have no actual birth date. The entire narrative of his supposed life is definitely false, even if he was real. It was compiled from a bunch of different writings about him by preachers who never actually claimed to have met him (except through “visions” and the like). The writings themselves are conflicting and are more likely the result of an attempt to construct christian mythology out of nothing.
B) I have yet to see one real m’kor, but if we’re so dedicated to rejecting our religion because of a christian myth, then maybe I’ll go out to mickey d’s tonight instead of having a Shabbos meal.
C) There is nothing that the Yeshu (if he was real) would have loved to see more than Jews being m’vatal Torah in his honor. That’s pretty much everything he stands for in the new testament. Maybe this “minhag” is the result of christian missionaries forcing Jews to not learn as part of one of the many historical campaigns against the Jewish religion.