Autumn is one of the four temperate seasons. Autumn marks the transition from summer into winter usually in September Northern Hemisphere.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. The actual lag varies with region. Some cultures regard the autumnal equinox as "mid-autumn", others with a longer lag treat it as the start of autumn. Meteorologists (and most of the temperate countries in the southern hemisphere) use a definition based on months, with autumn being September, October and November in the northern hemisphere.
In North America, autumn is usually considered to start with the September equinox. However, according to the Irish Calendar which is based on ancient Celtic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. According to United States unofficial tradition, autumn runs from the day after Labor Day (i.e. the Tuesday following the first Monday of September) through Thanksgiving (i.e. the fourth Thursday in November), after which the holiday season that demarcates the unofficial beginning of winter begins.