There may already be pumpkin spice everything for sale and decor of Halloween, pumpkins and even turkeys have been on the shelves for weeks now, but technically Fall doesn't start until 3:02 p.m. today, September 22, 2017.
So what does that mean exactly? September 22 is called the "Autumnal Equinox". On this day the sun hits directly along the equator (at 3:02 p.m. CST to be exact) and evenly distributes sunlight to both the northern and southern hemispheres. The earth sits at a 23.5 degree tilt. As we move in time from September through December, the earth will continue to tilt towards the southern hemisphere, bringing less sunlight to the northern hemisphere (i.e. cooler temperatures for us during the winter months).
From this description, you can probably conclude that the Autumnal Equinox is an astronomical season, and not a meteorological season. While meteorologists (like myself) will acknowledge this day as the "first day of Fall", you may also hear meteorologists talk about something called "meteorological Fall" That day already occurred on September 1st. The meteorological seasons are marked as such:
First day of Fall - September 1st
First day of Winter - December 1st
First day of Spring - March 1st
First day of Summer - June 1st
The difference between the two, is that these seasons are based off of the annual temperature cycle, rather than the astronomical cycle.