It's really simple. The simple, short answer is that they didn't. The celebrated the solstice on the solstice. The problem is that you aren't using the same calendar. Christmas has been celebrated on December 25 ever since the early 4th century, around 300 years after the crucifixion. But what calendar was used in that region at that time? The Roman calendar. The Roman calendar had the equinox on December 25, see Bradt, Hale, Astronomy Methods, (2004), p. 69.Meili wrote:Why did the Pagans celebrate a few days after the solstice?
The Roman calendar eventually became the Julian calendar and was refined until it was replace with the Gregorian calendar in the late 16th century.