Today is the first day of spring in the Northern Hemisphere, the vernal equinox, and the first day of fall in the Southern Hemisphere, the autumnal equinox. However, whichever side of the equator you're on, we both share one fun fact: it's the only two days of the year (in March and September) when the sun rises due east and sets due west, worldwide. Here's some fascinating information, if you want to learn more.
Our days and nights are also of (almost) equal length, which is where the name equinox comes from: the Latin words aequi, which means "equal," and nox, which means "night." That link will also tell you about how the Mayans celebrated the equinox, which I found interesting but also a little scary.