September has 30 days, and its name is a fossil from an older Roman calendar: it comes from the Latin “septem,” meaning seven, because September was originally the seventh month of the year in a calendar that began in March. When January and February were later added to the front of the year, September shifted to the ninth position, but it kept its old numerical name — the same pattern explains October, November, and December.
In the Northern Hemisphere, September brings the autumn equinox in its final days, when day and night are roughly equal before nights start growing longer — the traditional start of fall. In the Southern Hemisphere, that same equinox marks the beginning of spring, as days lengthen and temperatures start rising.
September is strongly associated with new beginnings in many Northern Hemisphere countries, since it’s when the school year typically starts after the summer break. It’s also a month of harvest in agricultural traditions across the Northern Hemisphere, as summer crops are gathered before the colder months set in.