The birth calendar explanation seems most likely to me.
I have always heard about it borrowing pagan festivals, but the thing is, it would be one heck of a conspiracy to be the case without any mentions of it early in Christian history.
It is specifically mentioned in early christian history. The author is saying that he believes he has evidence showing that Christ’s birth was already being celebrated on December 25th before the assimilation of pagan holidays and rituals took place.
Christian’s celebration of the Resurrection became all about baby chicks and bunnies, symbols of the spring season of fecundity (oestrus)as celebrated by pagans. Most confusing, and quite needlessly so.
Roman Catholic missionary priests have been quite adept at laying a veneer of Christian symbolism on pre-existing festivals celebrated by pagan peoples they ostensbily were trying to convert to Christianity. Celebratory parades observed by indigenous Latin American Indians and succeeding mixed breed peoples (Spanish influence) are examples.