When did pope Julius declare Christmas?
Neither pope Julius nor declare. The truth is Pope Telesphorus in 126-136 AD institutionalized the long-standing traditional Christmas midnight mass, which had been informally celebrated, following examples from the angels (Luke 2: 13-14) and the shepherds (Luke 2: 20).
And suddenly there was with the angel a multitude of the heavenly host praising God, and saying, Glory to God in the highest, and on earth peace, good will toward men. (Luke 2: 13-14)
And the shepherds returned, glorifying and praising God for all the things that they had heard and seen, as it was told unto them. (Luke 2: 20)