I just bought my ticket for my 2 month (28 Jan-1 Apr) volunteer vacation in Mexico next year. I'll be working on an environmental project in the Yucatan for 6 weeks inFebruary and March and have 2 weeks after to travel as I decide.
I know that will include visiting Chichen Itza on the Vernal Equinox to see the Serpent's shadow on el Castillo.
I've heard that the plaza is packed with tourists on the equinox but empty the day after. That's when I'll go.