Autumn. 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 or March. 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 and June. 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. 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 Hallowe'en, the approximate mid-points between midsummer, the autumnal equinox, and midwinter. Meteorologists 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 and ends with the winter solstice. 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. After the Greek era, the word continued to be used as the Old French word or 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. Before the 16th century, harvest was the term usually used to refer to the season, as it is common in other West Germanic languages to this day. However, as more people gradually moved from working the land to living in towns, the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, and autumn, as well as fall, began to replace it as a reference to the season. The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, with the Old English or and the Old Norse all being possible candidates. However, these words all have the meaning to fall from a height and are clearly derived either from a common root or from each other. The term came to denote the season in 16th-century England, a contraction of Middle English expressions like fall of the leaf and fall of the year. Compare the origin of spring from spring of the leaf and spring of the year. During the 17th century, Englishmen began emigrating to the new North American colonies, and the settlers took the English language with them. While the term fall gradually became nearly obsolete in Britain, it became the more common term in North America. The name backend, a once common name for the season in Northern England, has today been largely replaced by the name autumn. 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. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains that ripen at this time. Many cultures feature autumnal harvest festivals, often the most important on their calendars.