On March 21st and September 21st you can witness the incredible accuracy of Mayan astronomy as it was integrated into architecture. The Mayas planned their lives around the sun; their daily lives centered on the sowing and harvesting of their crops, especially corn. At the spring equinox they planted the crops, and the fall equinox was their signal to begin the harvest. The Mayas built complex structures using advanced geometry and astronomy to map the cycles of the sun. The equinox is an event of international importance, so be prepared to deal with crowds, parking, and walking. This is a sacred time for many, so please be respectful of those who are meditating and the Mayas who are doing their ceremonies.
The equinox phenomenon can actually be viewed in Chichén Itzá on the El Castillo pyramid for four days, so if it is raining or cloudy or you just can't get there, it is visible from the 20th - 23rd of March and from the 21st - 24th of September. During the equinox the sun casts its rays on the pyramid, forming seven isosceles triangles that resemble the body of a serpent 37 yards long slithering downwards until it joins the huge serpent's head carved in stone at the bottom of the stairway. It is said this snake is trying to make it to the well of sacrifice which is in the same direction. Although the site normally closes at 5 pm, they will stay open a bit longer to be sure you don't miss the end of the event.
At the Mayan site of Dzibilchaltún, March 21/22 and Sept. 21/22 at 5 am is the day and time of the equinox when the sun sends its beams through the two windows of the Temple of the Seven Dolls providing a lovely spectacle of Mayan exactitude. The site opens at 4.30 am on those dates.
Dzibilchaltún is located 16 km north of Mérida and while it doesn't have the giant pyramids of Chichén Itzá and Uxmal, it does have its own special claims to fame such as its "museum of the people", Museo del Pueblo, the refreshing cenote where you can take a swim, and the Franciscan chapel that blends in with the archaeological site.
For Dzibilchaltún, you should try to get there just before sunrise since the event (equinox show) happens at 5:00 am and lasts only a few minutes.
For Chichén Itzá, you want to be there some time before 3 pm. The show will last for about an hour. Be prepared to share the event with a huge crowd.
Here are two videos produced by Centro Sak Beh about the significance of the equinoxes and solstices: