10 But the angel said to them: “Have no fear, for, look! I am declaring to YOU good news of a great joy that all the people will have, 11 because there was born to YOU today a Savior, who is Christ [the] Lord, in David’s city. 12 And this is a sign for YOU : YOU will find an infant bound in cloth bands and lying in a manger.” 13 And suddenly there came to be with the angel a multitude of the heavenly army, praising God and saying: 14 “Glory in the heights above to God, and upon earth peace among men of goodwill.”
OK, is this proof enough? This comes straight from Luke 2 right from their own Bible! Notice the celebration right in heaven when Jesus was born. Is this not what Christmas means? And so what if the date got confused by the early Catholic church? They are still celebrating in this fashion, though with material things (angels have no material needs) like Christmas trees and gifts.