The first day of spring in the Northern Hemisphere was on Sunday, March 20. The day is also referred to as the vernal equinox (sometimes also referred to as the spring equinox or March equinox) and it is almost always either March 20 or March 21.
It is the only time of year when the sun rises in the east and sets in the west for everyone across the world. It’s also the only moment each year that the Earth’s tilt is zero in relation to our sun. So, if you were standing on the equator, the sun would pass directly over your head. The vernal equinox is when plane of the earth’s equator, making night and day approximately equal lengths all over the world. On that day, both of Earth’s hemispheres get an equal amount of sunlight.
There are actually two ways to determine the first day of spring — the astronomical cycle or the meteorologic cycle — but most people use the astronomical cycle. The astronomical cycle is always based on the March equinox, whereas the meteorologic cycle bases its first day of spring on seasonal weather and temperature patterns.
The date varies slightly because the Earth’s orbit is constantly changing in relation to the sun, while at the same time the gravity of other planets impacts the Earth’s location in space. Those physical dynamics, coupled with the fact that each calendar year always has a different number of days (think leap years), mean that the first day of springs varies slightly from year to year.
The spring equinox in the Southern Hemisphere happens at the exact opposite time of the year, so it is the fall equinox for people on the other side of the world (the same way that winter and summer are reversed for both hemispheres).