The first day of autumn is in 2015 on September 23 (according to the astronomical definition). It is also called the autumnal equinox, fall equinox or the beginning of autumn. Autumn is one of the four temperate seasons, the transition period between summer and winter usually in September (Northern Hemisphere) when the arrival of night becomes noticeably earlier.
The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. According to United States unofficial tradition, autumn runs from the day after Labor Day through Thanksgiving, after which the holiday season that demarcates the unofficial beginning of winter begins.
Association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. Many cultures feature autumnal harvest festivals, often the most important on their calendars. Still extant echoes of these celebrations are found in the mid-autumn Thanksgiving holiday of the United States and Canada, and the Jewish Sukkot holiday. In the Northern Hemisphere, autumn is also associated with the Halloween season.
Autumn in poetry has often been associated with melancholy. The possibilities of summer are gone, and the chill of winter is on the horizon. Skies turn grey, and many people turn inward, both physically and mentally. Furthermore, autumn has a strong association with American football, as the regular season begins during September and ends with playoff competition in December or January, in the winter season
Note: The date September 23, 2015 is calculated for Eastern Standard Time (UTC-5). The exact date for autumnal equinox can vary in other time zones.