The vernal equinox hits the Earth at 11:44 UT (07:44 Eastern US time) Friday, March 20th! What that means to most people is that it's the first day of spring (or fall in the southern hemisphere), but they're wrong.
What it means in real life is that there is very roughly equal amounts of daytime and nighttime (the word equinox means "equal night"); it's not exact due to the Earth's atmosphere, but that's neither here nor there.
For astronomers, it means the center of the Sun has 0° declination, and it crosses from the southern celestial hemisphere into the northern one.
For NASA, it means they can celebrate Sun-Earth day.
And for some people, it means it's the only time they can stand an egg on its end. They're wrong too:
For me, it means an easy blog post for the day. So happy equinox!