I’ve made the point before, especially to Adventist, “Don’t claim you’re not a Sunday observer if you observe Christmas!”
“The Date of Christmas. The adoption of the 25th of December for the celebration of Christmas is perhaps the most explicit example of Sun-worship’s influence on the Christian liturgical calendar. It is a known fact that the pagan feast of the dies natalis Solis Invic ti—the birthday of the Invincible Sun, was held on that date.71 Do Christian sources openly admit the borrowing of the date of such a pagan festivity? Obviously not.72 To admit borrowing a pagan festival, even after due re-interpretation of its meaning, would be tantamount to an open betrayal of the faith. This the Fathers were anxious to avoid.