(I never could wrap my head around all this. I had enough problems with Easter events, where the math makes a detour through a Lunar calendar.)
EDIT: And my memory of the Tradition is wrong too, it's supposed to be 24 - as confirmed by https://news.ycombinator.com/item?id=45710618, and corroborated the two "Paw Patrol" themed Advent Calendars I just found still stashed in my home office.
It's actually the March equinox. "Spring" is true only in the northern hemisphere. What's more it's the ecclesiastical equinox, not the astronomical equinox, whose date actually varies depending on the year.
Never mind that all this is descriptive of dating in countries that grew up with Western Christianity. Countries where Eastern traditions dominate often date it differently.
(I never could wrap my head around all this. I had enough problems with Easter events, where the math makes a detour through a Lunar calendar.)
EDIT: And my memory of the Tradition is wrong too, it's supposed to be 24 - as confirmed by https://news.ycombinator.com/item?id=45710618, and corroborated the two "Paw Patrol" themed Advent Calendars I just found still stashed in my home office.