I give a bit of a defense of celebrating Christmas here: https://www.sovereignkingchurch.com/sermons/christmas-cursing-and-blessing?fbclid=IwAR3fUeiZgTbHhJth_vPU5uYVIt5HGGCqhf3QImr_9lR6ChHnQA7xq99JpH8
It’s a portion of this sermon so it may not answer every objection.
But to get to your main objection: does the church have the authority to establish Christmas?
It has the responsibility to teach the whole council of God. With that responsibility comes to the authority to select what part and when it may do so. I don’t see how it is a violation of the regulative principle to set aside a time to preach and teach on the birth of Christ or to rejoice in his first coming.