Lots of people claim many dates that Christians adopted are from ANY previous source in order to disprove Christianity or weaken it. For instance have a look at this:
Influence on Christianity
During the last few decades, many Christians have re-examined the Jewish roots of their faith. As a result, some have increasingly become aware of and adopted some aspects of some Jewish holiday, but within a Christian theological framework. As such, some Christians light the Hanukkah menorah and observe other Hanukkah rituals. In addition to this, some allege a connection between the miracle of the light being given in the Temple and the birth of Jesus (whom they call “the Light of the world”). Some hold that Hanukkah (the 25th of Kislev), not December 25, is actually the proper date for celebrating the birth of Jesus. Thus they combine or conflate the two celebrations (Hanukkah and the birth of Jesus), while others continue to keep them separate and celebrate both Hanukkah and Christmas.
I’m not supporting this theory, just throwing it out there for you to think about. Be aware of anyone that alleges anything without showing you proof.