Spring begins this year on March 19. If you want to be specific, it begins at exactly 11:06 p.m. We don’t really think about what this means, just that Winter has ended and (hopefully) warmer weather will arrive.

But what exactly does Spring mean, and why isn’t on the same day each year?

If you want the science-y answer, the first day of Spring is called the vernal equinox. This is when approximately 12 hours of daylight is observed. The amount of daylight gradually gets longer until the first day of Summer.

According to the National Day Calendar, the vernal equinox marks the moment the sun crosses the celestial equator. This is the imaginary line in the sky above the Earth’s equator, from south to north. The vernal equinox happens on March 19, 20, or 21 every year in the Northern Hemisphere. In the Southern Hemisphere, this same event marks the beginning of fall.

So why doesn’t Spring begin on March 21 like it used to? The answer has to do with the calendar we use. According to almanac.com, for most of the 20th century, Spring began on March 21. However, it begins earlier and earlier because of the 400-year Gregorian calendar. The last time Spring began on March 21 was 2007, and that was the last time Spring will begin on that date for the rest of our lives.

Other fun facts about Spring:

• On March 19, the sun will rise due east and set due west, the only day of the year that it does that.
• The sun rises and sets faster at the equinox.
• The word “equinox” means “equal night” in Latin. Night and day are almost equal times, but not quite.
• In most Christian denominations, Easter is celebrated on the first Sunday after the first full moon after the vernal equinox. Because of this, Easter can fall anywhere from March 22 to April 25.
• All of the solar system’s planets also have equinoxes.

The one thing that everyone realizes about Spring, though, is that it’s a time to go outside and enjoy the weather and sunlight. Except, of course, if you have allergies, but that’s another story.