(The Julian calendar aligns with the Roman calendar of the first century. The proleptic Gregorian would be two days off from that in that century. There's no issue which one you use, but IMHO most historians use the Julian because the locals did at that time.)
The leap year drift does account for why Christmas is December 25 instead of the solstice around December 21, because it took about 4 centuries of drift to solidify; but the solstice has always been in late December and not at the beginning (kalends) of a new Roman month. But God knows why he let different eras use different calendars, and no data point is contradicted by the existence of drift of "Christmas".
(The Julian calendar aligns with the Roman calendar of the first century. The proleptic Gregorian would be two days off from that in that century. There's no issue which one you use, but IMHO most historians use the Julian because the locals did at that time.)
The leap year drift does account for why Christmas is December 25 instead of the solstice around December 21, because it took about 4 centuries of drift to solidify; but the solstice has always been in late December and not at the beginning (kalends) of a new Roman month. But God knows why he let different eras use different calendars, and no data point is contradicted by the existence of drift of "Christmas".