First, I’m not going to be answering that question, I’m asking it, and arguing why it should have been on the Solstice (when being setup, not now), and not some random day after. This isn’t some cute way of raising the question then providing the answer, such as “why is it dark at night?” Which while it may seem a clear answer, we’re on the shaded side of the Earth from the Sun. But given the number of starts out there, that every spot you look at has tons of galaxies, even in the gap of stars, the night sky should at worst look like the Milky Way all over the place, to being even a bit brighter, even on a night with no moon overhead. There would still be night, it would still be “dark”, but it should be brighter than it is. There is of course a rather simple answer, and I’ll let you go about exploring that on your own.
This is just a simple question of why didn’t the Gregorian Calendar set January 1st on the Northern Hemisphere Winter Solstice? Why some random date 11 days later? Now, it’s not always 11 days, sometimes it’s plus or minus one, but in general, the Winter Solstice is December 21st in the Northern Hemisphere, where the calendar was based.
One could argue that it should be on one of the Equinoxes, to be more fair to both hemispheres, but that would have been a far bigger shift from the Julian. Also I rather like the idea of a Solstice change, going from darkness into light. It could have carried more religious significance. Christ is the Light of the World, and all that jazz.
The Gregorian Calendar is mostly set up around Easter, but Easter doesn’t fall on a specific date, it floats. So moving January 1st to the Solstice wouldn’t have changed that float. Instead of being late March to anywhere in April, Easter would have been anywhere in April to super early May at the latest, and that would have been ultra rare, far less common than Easter being in March now. It would have largely fixed Easter to April alone. Instead of a month named for Aphrodite, they could have named it Ostara, the Celtic Goddess from which almost all the Easter traditions, such as rabbits, and the Easter Egg, come from, they already have most of the days and months named after various pagan gods/goddesses, so what’s changing the name from one to another? You now have your Spring Equinox on April 1st, all the more reason to change April’s name to Ostara, all due respect to Aphrodite. Heck, ask most people today what pagan gods the months and days were named for, and you’d likely get a blank look, and likely never knew the origins of the names. [For those not in the know, March = Mars, April = Aphrodite, May = Maia, June = Juno… then they renamed Quintillis (quin being five) and Sextilis (sex being six) July and August respectively, but then we stick with the old names of September (sept being 7… and yes, September is now the 9th month, and October the 10th, but one upon a time September, October, November and December all fell on the 7, 8, 9 and 10th months… don’t get me started on how we call billionaire, trillionaire and the rest on up from there wrong here in the US… and recently in the UK), October (oct being 8… octopus), November (nona being 9), December (deca being 10… decimal). January is ianua, or latin for door, as it is the door into the new year), February is februum, or purification as there was a day of purification in the Roman system there)…. Sunday is named for the Sun, Monday for the Moon, Tuesday is for Tyr (Norse god of combat), Wednesday is of course for Odin, Thursday for Thor, Friday is for Frigg (another Norse goddess), Saturday for Saturn (the god, not the planet)… and no, I don’t have all those 100% memorized, like for some reason I keep forgetting Tuesday and Friday’s gods]. April/Ostara would have been the one month Easter is pretty much guaranteed to have Easter in, heck, make it 31 days and it always would have had Easter, at least using the methods they use to determine Easter (which Biblically should be based on the Jewish Passover, which it comes close to now, but doesn’t follow the way it should). Anyhow, the point being, is that it really wouldn’t have changed Easter in any way, shape, or form, save lock it into one single month every year. So they still could have done it.
While such a start date doesn’t fix some of the odd problems the calendar has, and there would still be efforts to reform the calendar, at least January 1st would have made sense… of course then you get into the argument of do you put the 1st on the Solstice itself, or the day after, which is the first full day of the sun appears to be moving North again? I’d say the day of, it gives the day an extra bit of reason to celebrate. So the question still remains, why didn’t they set January 1st on the Solstice?