in addition to giving presents being chukas hagoyim,
the 25th day of the month of December (whichever way you want to call it)
represents the day that Yosef HaTzaddik went to eshes potifar “laasos tzurcho ima”–this day was chaf hey/ the 25th of the month, and it was the day that the entire mitzrayim annually went to serve avodah zara (this is why potifar wasnt around).
so the 25th day is steeped in tumah since the days of mitzrayim.
it also says, if im not mistaken (and please correct me if i am), “Kihayom hazeh” in the passuk when it describes Yosef going to eshes potifar–“KiHa”=chaf hey=25