The idea , particularly popular in the 18th and 19th centuries , that the date of 25 December for Christmas was selected in order to correspond with the Roman festival of Dies Natalis Solis Invicti , or `` Birthday of the Unconquered Sun '' , is challenged today .
The Romans held a festival on December 25 called Dies Natalis Solis Invicti , `` the birthday of the unconquered sun . ''