The Spring Equinox - this year, on Tuesday, March 20th - inspires many traditions over the world, though few are as exciting as the illumination of the Mayan pyramid of Chichen Itza in the Mexican Yucatán. An equinox occurs twice a year (for Spring and Winter), when the sun appears precisely over the equator, which creates a day and night which are exactly the same length.
The Mayan ruins of Chichen Itza is one of the best places in the world to appreciate this seasonal change, as the Mayans built their pyramid to project light during both of the equinoxes. At sunset, the light projects the image of a snake slithering down the side of the pyramid, which takes about 45 minutes. If you're looking to celebrate closer to home, look for festivals and celebrations in parks, gardens and other public places. If nothing official is happening in your city, even an afternoon or evening set in a park to appreciate the changing of the seasons is worthwhile time spent.
For images of the spring equinox being celebrated across the world, the National Geographic has some amazing captures from their 2009 edition, along with a great article on myths and facts about the equinox.