Do you know why Christmas isn't on the Solstice?
It's a calendar thing. When Julius Caesar set up the Julian Calendar in 48BCE, the spring equinox was set as 25 March, which would usually put the winter solstice on 25 December.
Because the solstice was on the 25th, so Christmas came to be on that same date.
The Julian Calendar drifts by about three days every four hundred years against the actual orbit of the earth.
So, when the Christian churches met at İznik (then Nicaea) in 325 CE to settle the date of Easter, they chose to have the Spring Equinox on 21 March, because that is when the Equinox was in 325. Easter's calculation depends on the equinox.
1200 years later, when someone finally decided to do something about the drift in the Julian calendar, the priority was to get Easter aligned back with the sun and the seasons, and that meant getting the equinox back to March 21.
So the Gregorian calendar in 1582 CE was set to put the equinox on March 21 and therefore the winter solstice on December 21.
So that's how we ended up having a solstice festival four days after the solstice.
