One sometimes sees the claim that the date of December 25, and sometimes other aspects of the Christmas celebration, were borrowed from pagan cults contemporaneous with early Christianity. Saturnalia, the feast of Sol Invictus, and the birth of Mithras, among others, have been named as precursors/models for Christmas. The reason given for the borrowing also varies: as a way to entice potential converts, a way to blend in and avoid persecution, or a natural process of syncretism and cultural evolution.

Arguments about this are very common on the internet, but very rarely, in my experience, does anyone cite what looks remotely like a reliable source. What do we really know about the origins of the Christmas holiday and its relationship to the various pagan holidays that were celebrated around the same time of year? Are there other reasons that specific date might have been adopted?