Oxford Dictionary of Archeology: Solstice: “Literally, the sun’s standstill ( on a sundial ) . The extreme positions of the sun at midwinter when its eastern risings and western settings appear to take place in the same position on the horizon for ——THREE DAYS—— in succession.” Those 3 days are currently Dec 22, 23rd, and 24th.
Depending on calendarial shifts, December 25th was when the sun would make it’s way into Capricorn by 1 degree along the ecliptic. Since this coincided with the sun’s first ascent Northward, it marked the occasion.