When is the first day of spring?

The first day of a calendar spring is a fixed date that invariably falls on March 21. The date of the beginning of the astronomical spring, which depends on the movement of the Sun, is movable. Astronomical spring begins on the equinox, otherwise known as the spring solstice. This is the time when the day and night are exactly 12 hours each. In 2021, the first day of astronomical spring falls on March 20.