It's nearly scarf and flannel wearing weather, with the official first day of fall Saturday September 22 at 10:49 a.m. EDT. This pertains to the Northern hemisphere and is known as the Autumn Equinox.
What is an Equinox?
An equinox occurs twice yearly around March 20 and September 22 when the Earth's axis is on a tilt that is neither away from nor towards the sun. The center of the sun is the same plane as the Earth's equator.
The Autumnal Equinox occurs at the moment the sun crosses the celestial equator. The celestial equator is the line in the sky above the earth's equator that goes from north to south. The date of its occurrence is always between September 22-24.
During the two equinoxes every year in March and September the length of day and night are almost exactly the same. In the Northern hemisphere the September equinox is know as the "autumnal (fall) equinox" and in the southern hemisphere, where seasons are opposite, it is known as the "spring (vernal) equinox"
Not until after the autumnal equinox do we see the days with exactly 12 hours between sunrise and sunset which can occur anywhere from September 25-28.
In 2013 the first day of fall will be the same September 22 and in 2014 and 2015 it will be on September 23. However, just because the calendar says its fall that doesn't necesarilly mean it will feel like fall in full swing. Natural factors and global warming have to do with changing patterns in temperature and mild weather so we might have to wait to see fall leaves in full effect.