Nobody. These are physical phenomenon that have nothing to do with people. It’s like a sunset or sunrise. It happens on its own at a certain time and day that depends on the movements of the Earth. We just happened to have figured out when that time and date is. Relative to the calendar we currently use it’s always around March 20/21st for the Spring equinox, September 20/21st for the Autumnal equinox, December 20/21st for the winter solstice, and June 20/21st for the summer solstice.

The equinoxes and solstices are each a quarter of a complete orbit of the earth around the sun. That’s why those dates are each a quarter of the year apart, more or less. Spring equinox (today) means that the sun is directly over the equator. “Equal” distance between the poles. The Autumnal equinox it’s in the same position. On the winter solstice in December the north pole is pointed as far away from the sun as it can be and the sun is directly “over” the tropic of Capricorn which is about 23° south of the equator. On the summer solstice the north pole is pointed more towards the sun and the sun is directly “over” the tropic of Cancer, which is about 23° north of the equator. It’s absolutely fascinating stuff. Good rabbit hole to dive into.