At 18:26 Universal (Greenwich) time today, March 20, the center of the Sun will be at Right Ascension 0h0m0s and a declination of 0o0m0s, which means it will pass directly over the Earth's equator as it moves north in its annual path around the ecliptic.
You and I call that the vernal equinox. OK, I call it that. You'd probably call it the first day of Spring. I wouldn't call it that, but hey, semantics.
Around the United States today, kids in their classrooms will be standing eggs on end, or at least newsrooms will be talking about it. If you have no idea what I'm talking about, then read this, and then this, and if you're really enjoying yourself, this.
And if you are really a glutton for punishment, see when the equinoxes, solstices, perihelia, and aphelia will be for the next 14 years.