Until the Gregorian calendar there were constant shifts of the calendar in relation to astronomical events. With the introduction of the Gregorian calendar, a shift of many days was introduced, but was this shift enough to correct the shift of the solstice to its original date at the time of Caesar (or before)? I have heard that the solstice was originally at 23 Dec, or even the 25th. Related to this, I have heard that 25 Dec was a festival of Sol Invictus in ancient Rome. Are there sources for this? Was the date the date of the solstice, or was it a few days later, when the days were noticeably lengthening?
I've written elsewhere about the Roman festival of the Sun on December 25th (https://www.reddit.com/r/AskHistorians/comments/6xbtao/was_the_birth_date_of_sol_invictus_influenced_by/dmf21c4?utm_source=reddit-android), and I'll happily go into more detail if you'd like.
I think it's important to point out that there is little to no evidence that the Romans worshipped the Sun at either the solstices or equinoxes, regardless of whether the calendar shifts had an impact. The festival on December 25th seems to have been a 3rd or 4th-century addition to the calendar, as it does not feature on any earlier one.