I wonder how to calculate the date of the vernal equinox, summer solstice, autumnal equinox, and winter solstice in Mathematica. I searched Wolfram Community for a post showing how to calculate the solstices and equinoxes but I couldn't a post. Kenneth R. Lang states in his book Essential Astrophysics

As the Sun moves along the ecliptic, it crosses the celestial equator twice, on its way north at the Vernal Equinox, on about March 20, and then at the Autumnal Equinox on about September 23. On either equinox, the Sun lies in the Earth’s equatorial plane, so the twilight zone that separates night and day then cuts the Earth in equal parts and the days and nights are equally long. The point at which the Sun is farthest north, is the Summer Solstice (on about June 21), and its most southerly point is the Winter Solstice on about December 22. The days in the northern hemisphere are the longest on the Summer Solstice, and shortest on the Winter Solstice. So the crossing of the Sun at the equinoxes and solstices mark the beginning of the seasons in the Earth’s northern hemisphere, and the location of these points on the celestial sphere are given in Table 1.1. I understand autumnal equinox is when the days are the same length.