What is the Spring Equinox?

The spring equinox marks the astronomical beginning of spring in the Northern Hemisphere. The balance of daylight shifts, marking the start of warmer, longer days. In the Southern Hemisphere, it marks the start of autumn. Equinox means equal night.

At the spring equinox, the sun is directly above the equator, and day and night are equal in length all over the globe.

What's the difference between astronomical spring and meteorological spring?

The exact date of astronomical spring ( marked by the equinox ) changes slightly each year for several reasons:

The Earth's orbit around the sun is not a perfect circle.
The Earth's tilt has an effect.
The Gregorian calendar leap year system is not a perfect match for the Earth's orbit.
Leap years shift the exact date of the equinox.

The spring equinox can be March 19th, 20th or 21st.

Meteorological spring starts on March 1st and ends on May 31st.