Actually it is the Fall Equinox, the point where the daylight and dark hours of the day are equal. <G>
Now they call it the first day of fall, actually it is the middle of the astronomical autumn.
The equinoxes and solstices divide the year into 4 segments that traditional were the midpoints of the seasons. The points when the relative position of the sun 'changes' and can be seen with even primitive key stones like StoneHenge and so on.
So the "start" is just an easy way to say the equinox is here. <G>
Pati, in Phx