Christmas Day used to be known as Mithras Day back in Rome, and it was meant to coincide with the Winter Solstice like Yalda Night in Persia (they're essentially the same holiday, same roots, same deity). But they moved it to December 25th to coincide with Dies Natalis Solis Invicti (Birthday of the Unconquered Sun) which was already established and actually used to be on the solstice but had shifted due to astronomical drift.