Gotta read this one to the end.
Check it out:

One of the most popular theories and commonly taught explanations for why Christmas is on Dec. 25th is because the early church placed Christian holidays at times of Roman celebration to co-opt the local pagan festivals.

Christians placed Christmas on Dec. 25th to co-opt Saturnalia, the mid-winter festival, or possibly the Festival of the Unconquered Sun — Sol Invictus. The theory went that Christians could get the heathen to convert by co-opting their own holidays.

There is one problem — it sounds more convincing than it is.