A: The first day of fall occurs at the autumnal equinox: Monday, September 22, 2008, at 11:44:16 EDT (15:44:16 UCT). This is the astronomical definition of fall, and is generally used when we refer to "fall" or "autumn."
It should be noted that meteorologists tend to refer to September, October and November as fall, which means fall starts on September 1.
In temperate and polar regions generally four seasons are recognized while in tropical or subtropical regions seasons are defined by wet, dry, and sometimes cool. These meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. The equinoxes and solstices mark the start of astronomical spring/autumn and summer/winter. The divisions are based on the rotation of the earth with respect to the angle of the earth. As the earth goes around the sun, picture it looking down on its orbit from above. summer solstice is when the earth's north pole is pointed most directly towards the sun (longest day) - warming the northern hemisphere. Winter solstice is when the earth's south pole is most directly pointed toward the sun warming the southern hemisphere and leaving the north cold (shortest day). The equinoxes are the midway points between the solstices with the sun halfway between its northmost and southmost points, so we have equal day and night.
- z monkey
[as found on WikiAnswers]