I’m just learning of this and was studying an Rorate Caeili article on the Ember Days this afternoon taking notes for literacy on this lovely subject.
Which is turn led me to find out that Solstice (beginning of Summer and Winter) is when the sun is at its northern (southern) most point.
Makes me realize my continuing ignorance and that I can still learn. As I thought the beginning of Fall, Winter, Spring and Summer were just dates on the calender.