Today, March 20, is the first day of Spring in the northern hemisphere marked by the vernal equinox.
The equinox is cool because it's a time of year when the night and day are almost equal.
This awesome GIF of Earth from NASA's Astronomy Picture of the Day (APOD) helps to explain why that happens.
Generally throughout the year, the changes in season are caused by the Earth's tilt on it's axis. When the top of the Earth is titled toward the sun, it's summer in the north. There's more sunlight over that part of the world and the days are longer. At the equinoxes, the Earth's axis doesn't point toward or away from the sun.
The GIF below starts at the equinox in September 2010. The line that separates day and night, called the terminator, is vertical so that half of Earth is in darkness and half is in the sunlight. Halfway through the GIF, Earth reaches the equinox in March 2011 causing spring in the north. The GIF ends with the September equinox again.
Here's the full explanation from NASA's Astronomy Picture Of The Day (APOD):
From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the Sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere — and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken — and will take — around the Sun.
Watch the video, from APOD on YouTube: