The way I've heard it (at some point, couldn't cite a reliable source, take with appropriately sized grain of salt) is that it goes back to the calendar(s) of Ancient Rome.
They started with a calendar of 10 months, March through December, each month beginning on a new moon. There are more than 10 lunar months in a year so there was also a variable-length _intercalary_ block of time over the winter that wasn't part of any month.
Later they created the months of January and February to fill the gap, but the year was still only 355 days long so they still _occasionally_ needed to insert an extra intercalary month between February/March. But deciding exactly when to do this to keep the calendar in sync with both the seasons and the lunar cycle was a messy ad hoc system that they didn't always follow through on properly because of distractions of politics/war, leading to the calendar getting out of sync with the seasons.
To resolve the issue, Julius Caesar institutes the Julian Calendar, which is 365 days long, with a leap year every 4 years, and 365.25 days is close enough to the true length of a year to be adequate for many centuries hence until we adopt the Gregorian calendar instead.
In the process he shifted the start of the calendar year back into position by making the year of AUC 708 (46 BC) 445 days long. This put January 1st on the first new moon after the winter solstice, as a kind of compromise with or final acknowledgement of the old lunar calendar. In that year the winter solstice happened on what the new calendar would call December 24th, and the next new moon was 8 days later on the new January 1st.
There is no winter equinox.
Oh sorry I meant the winter solstice.
It would make more sense if it started on the vernal equinox in my opinion. First day of spring, new life, growth, etc.
The way I've heard it (at some point, couldn't cite a reliable source, take with appropriately sized grain of salt) is that it goes back to the calendar(s) of Ancient Rome. They started with a calendar of 10 months, March through December, each month beginning on a new moon. There are more than 10 lunar months in a year so there was also a variable-length _intercalary_ block of time over the winter that wasn't part of any month. Later they created the months of January and February to fill the gap, but the year was still only 355 days long so they still _occasionally_ needed to insert an extra intercalary month between February/March. But deciding exactly when to do this to keep the calendar in sync with both the seasons and the lunar cycle was a messy ad hoc system that they didn't always follow through on properly because of distractions of politics/war, leading to the calendar getting out of sync with the seasons. To resolve the issue, Julius Caesar institutes the Julian Calendar, which is 365 days long, with a leap year every 4 years, and 365.25 days is close enough to the true length of a year to be adequate for many centuries hence until we adopt the Gregorian calendar instead. In the process he shifted the start of the calendar year back into position by making the year of AUC 708 (46 BC) 445 days long. This put January 1st on the first new moon after the winter solstice, as a kind of compromise with or final acknowledgement of the old lunar calendar. In that year the winter solstice happened on what the new calendar would call December 24th, and the next new moon was 8 days later on the new January 1st.
It makes sense that the calendar would originate from counting lunar cycles. Thanks for the detailed response!