The first day of autumn falls on the autumnal equinox, which varies in date from Sept. 22 to Sept. 23, depending on the year. An equinox is when there is an equal amount of daylight and darkness in a day.
After the autumnal equinox, the daylight hours grow shorter, and the amount of dark hours each day grows greater. Autumn ends on the midwinter solstice, which falls on Dec. 21 or 22. The midwinter solstice is the day with the most dark hours in one 24-hour period and is the first day of winter. During the winter season, the daylight hours grow longer until it becomes spring on the vernal equinox.