Today is officially the first day of fall. Those with a penchant toward the romantic might prefer to call it the autumnal equinox, for the scientifically inclined, it's "a change in the orientation and angles between the Earth and its nearest star." Those in the Southern hemisphere, naturally, will refer to it as the first day of spring.
The season will shift at exactly 4:44 pm EDT, appropriately underscoring the symmetry of the day: as the sun passes along the equator, day and night will be about equal in length across the entire planet.
This NASA video capturing the changing seasons is actually from 2011. But arguably the nicest thing about the seasons is that they're constant: