Why December 25?

A nice overview as to why Christmas falls where it does: Why December 25? | Christian History:

The eventual choice of December 25, made perhaps as early as 273, reflects a convergence of Origen's concern about pagan gods and the church's identification of God's son with the celestial sun. December 25 already hosted two other related festivals: natalis solis invicti (the Roman "birth of the unconquered sun"), and the birthday of Mithras, the Iranian "Sun of Righteousness" whose worship was popular with Roman soldiers. The winter solstice, another celebration of the sun, fell just a few days earlier. Seeing that pagans were already exalting deities with some parallels to the true deity, church leaders decided to commandeer the date and introduce a new festival.