Astronomically speaking, the first day of spring is marked by the spring equinox, which falls on March 19, 20, or 21 every year. The equinox happens at the same moment worldwide, although our clock times reflect a different time zone. And, as mentioned above, this date only signals spring’s beginning in the Northern Hemisphere; it announces fall’s arrival in the Southern Hemisphere.
Interestingly, due to time zone differences, there isn’t a March 21 equinox in mainland U.S. during the entire 21st century! We won’t see a March 21 equinox again until 2101.
In the Northern Hemisphere, the spring equinox in the Northern Hemisphere (also called the March equinox or vernal equinox across the globe) occurs when the Sun crosses the celestial equator going south to north. It’s called the “celestial” equator because it’s an imaginary line in the sky above the Earth’s equator. Imagine standing on the equator; the Sun would pass directly overhead on its way north.
On the March equinox, the Northern Hemisphere and Southern Hemisphere receive roughly equal amounts of sunlight; neither hemisphere is tilted more toward or away from the Sun than the other.
The farmers almanac has an interesting article about the first day of spring. You can read it here:
