So I wonder who it was that discovered that the solstices and equinoxes were on the (roughly/approximately) 21st day of their respective months and not on the 25th day?
And when was this discovery made and implemented?
I think I may have found a partial answer to my questions.
Late last night when I should have been preparing for bed I ended up reading Wikipedia articles on the winter solstice, the Gregorian calendar, the Julian calendar, and the influential Roman writer Pliny.
Because it was so late, I unfortunately did not copy any urls, but I did learn that Pliny calculated the winter solstice to be December 25, when the sun entered the 8th degree of Capricorn. What I did not note is what is the 8th degree of Capricorn and why it was significant. I also didn't pick up on what influence Pliny may have had on solstice celebrations--or who figured out the solstice is December 21.
Pliny lived in the first century. He died trying to rescue people from the eruption of Mt. Vesuvius (AD 79).