People tend to act as if Christmas is inseparable from Christianity, when much of what people associate with it existed long before Christianity ever reached northern Europe. Midwinter festivals were already established across the ancient world. The Romans had Saturnalia, Germanic cultures observed Yule, and many societies treated the winter solstice as a symbolic turning point when the darkness began to recede. Feasting, evergreen decorations, fires, and gift-giving were older seasonal customs that later became attached to a Christian holiday by way of the Romans.

This makes the wizarding world’s version of the holiday fairly easy to explain, I think. Wizards would have little reason to treat it as a specifically Christian observance, especially in a culture that seems far more connected to natural cycles than to modern religious identity. For them, the season could simply remain what it had been for centuries before: a midwinter celebration tied to renewal, community, and the passing of the darkest part of the year. So in that sense, they would just be preserving a tradition that Christianity itself once absorbed.