September is the ninth month of the year in the Julian and Gregorian calendars, the third of four months to have a length of 30 days, and the fourth of five months to have a length of less than 31 days. In the Northern Hemisphere September is the seasonal equivalent of March in the Southern Hemisphere.
September marks the beginning of the ecclesiastical year in the Eastern Orthodox Church. It is the start of the academic year in many countries of the northern hemisphere, in which children go back to school after the summer break, sometimes on the first day of the month.
September (from Latin septem, "seven") was originally the seventh of ten months in the oldest known Roman calendar, the calendar of Romulus c. 750 BC, with March (Latin Martius) the first month of the year until perhaps as late as 451 BC. After the calendar reform that added January and February to the beginning of the year, September became the ninth month but retained its name. It had 29 days until the Julian reform, which added a day.
September begins on the same day as December in both common and leap years. No other month ends on the same day of the week as September in any year. This month and May are the only two months to have this property. In common years, September begins on the same day of the week as April and July and ends on the same day of the week as April and December of the previous year. In leap years, September starts on the same day of the week as January and October of the previous year and ends on the same day of the week as July of the previous year. In common years preceded by leap years, September begins on the same day of the week as January of the previous year. In years followed by common years, September begins and ends on the same day of the week as June and ends on the same day of the week as March of the following year. In years before leap years, September begins on the same day of the week as March and November and ends on the same day of the week as August and November of the following year.
Ancient Roman observances for September include Ludi Romani, originally celebrated from September 12 to September 14, later extended to September 5 to September 19. In the 1st century BC, an extra day was added in honor of the deified Julius Caesar on 4 September. Epulum Jovis was held on September 13. Ludi Triumphales was held from September 18–22. The Septimontium was celebrated in September, and on December 11 on later calendars. These dates do not correspond to the modern Gregorian calendar. In 1752, the British Empire adopted the Gregorian calendar. In the British Empire that year, September 2 was immediately followed by September 14.
September was called "harvest month" in Charlemagne's calendar. September corresponds partly to the Fructidor and partly to the Vendémiaire of the first French republic. On Usenet, it is said that September 1993 (Eternal September) never ended. September is called Herbstmonat, harvest month, in Switzerland. The Anglo-Saxons called the month Gerstmonath, barley month, that crop being then usually harvested.
The September equinox takes place in this month, and certain observances are organized around it. It is the Autumn equinox in the Northern Hemisphere, and the Vernal equinox in the Southern Hemisphere. The dates can vary from 21 September to 24 September (in UTC).
September is mostly in the sixth month of the astrological calendar (and the first part of the seventh), which begins at the end of March/Mars/Aries.
This list does not necessarily imply either official status or general observance.