Popular culture in the United States associates Labor Day, the first Monday in September, as the end of summer and the start of autumn; certain summer traditions, such as wearing white, are discouraged after that date. As daytime and nighttime temperatures decrease, trees change colour and then shed their leaves.Under the traditional East Asian solar term system, autumn starts on or around 8 August and ends on or about 7 November. In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September, October, and November. However, according to the Irish Calendar, which is based on ancient Gaelic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In the Irish language, September is known as Meán Fómhair ("middle of autumn") and October as Deireadh Fómhair ("end of autumn"). Persians celebrate the beginning of the autumn as Mehregan to honor Mithra (Mehr). Etymology
The word autumn () is derived from Latin autumnus, archaic auctumnus, possibly from the ancient Etruscan root autu- and has within it connotations of the passing of the year. Alternative etymologies include Proto-Indo-European *h₃ewǵ- ("cold") or *h₂sows- ("dry").After the Greek era, the word continued to be used as the Old French word autompne (automne in modern French) or autumpne in Middle English, and was later normalised to the original Latin. In the Medieval period, there are rare examples of its use as early as the 12th century, but by the 16th century, it was in common use.
Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September (Northern Hemisphere) or March (Southern Hemisphere). Autumn is the season when the duration of daylight becomes noticeably shorter and the temperature cools considerably. Day length decreases and night length increases as the season progresses until the Winter Solstice in December (Northern Hemisphere) and June (Southern Hemisphere). One of its main features in temperate climates is the striking change in colour for the leaves of deciduous trees as they prepare to shed. Date definitions
Some cultures regard the autumnal equinox as "mid-autumn", while others with a longer temperature lag treat the equinox as the start of autumn. In the English-speaking world of high latitude countries, autumn traditionally began with Lammas Day and ended around Halloween, the approximate mid-points between midsummer, the autumnal equinox, and midwinter. Meteorologists (and Australia and most of the temperate countries in the southern hemisphere) use a definition based on Gregorian calendar months, with autumn being September, October, and November in the northern hemisphere, and March, April, and May in the southern hemisphere. In the higher latitude countries in the Northern Hemisphere, autumn traditionally starts with the September equinox (21 to 24 September) and ends with the winter solstice (21 or 22 December).