We say that the autumn equinox happens on September 21, but this is an average, and it can actually happen the day before or the day after (the calendar is not that exact, which is why we have leap years to straighten it out every so often).

It is pretty easy to find out the exact date and time of the autumn equinox on the internet. The first day of fall is the next complete day. This is simple to apply if the equinox happens at 3 PM. But what if it happens at 3 AM? I’m afraid I don’t know the answer to that, but I’d bet people call the day stat starts at the next dawn the first day of fall.