Why was the first day of spring this year on the 20th...I thought it was always on the 21st?
The equinox occurs when the sun at noon is directly overhead at the equator. This location is called the 'sub solar' point. Changes in latitude can result in a different day when hours of daylight and darkness are equal. Although at the equator, the first day of spring marks the timing of equal hours of daylight and darkness, that isn't necessarily true for latitudes farther north. AND...because the length of our year...defined by our orbit around the sun isn't exactly 365 days...the exact timing of that event can vary. The insertion of leap years doesn't exactly account for that variation. That's why some years it's on the 20th, and some years on the 21st.