Today is pretty special. The day and the night are equal and lots of other weird things are happening too. Can you see on this pyramid there are kind of snake lines of sunlight going down the steps? That only happens on the Spring and Autumn Equinox, not on any other day of the whole year (hence all the people).
The autumnal equinox happens the moment the sun crosses the celestial equator, which is an imaginary line in the sky that corresponds to Earth’s equator. Every year this occurs on September 22, 23, or 24.
Here's the fascinating bit for this year: the full moon near the autumnal equinox is called the harvest moon; this year’s harvest moon on the evening of September 27 is a super moon that will also display a rare total lunar eclipse. The last time a total lunar eclipse coincided with a supermoon happened was in 1982, and it won’t happen again until 2033! Cool huh? Time to go outside and look up...