summer turning into autumn

Today I learned that the official start of our four seasons is not always on the 21st of their allotted months! I always thought September 21 was the Autumnal Equinox, December 21st was the Winter Solstice, etc.
Well the dates change depending on the position, or tilt, of the earth as it goes around the sun. The Vernal (Spring) and Autumnal Equinoxes happen when the sun is directly over the equator giving day and night of equal length. The Solstices are the days in June and December that give us the longest day and the longest night respectively.

For the year 2021, the Vernal Equinox was March 20, the Summer Solstice was June 20, the Autumnal Equinox will be tomorrow Sept 22, and the Winter Solstice will be Dec 21. At least the seasons change within a few days of the 21st, and not during different months like Easter does!