September is the ninth month on both the Gregorian and Julian calendars. The name September is derived from Latin word Septem which meant seven. On the old Roman calendar, this would be the seventh month since that calendar began in March. However, in 153 BC the Roman senate adapted a new calendar in which the year started in January and Septem became the ninth month. The Autumnal Equinox takes place between September 22-24 and inaugurating Autumn in the Northern Hemisphere. In the southern, the same equinox heralds Spring.
The Eastern Orthodox Church starts its liturgical year in September (they follow the Julian calendar instead of the Gregorian). The first full moon of September is often called the Harvest Moon since many farmers begin harvesting crops. The United States celebrates Labor Day on the first Monday in September. The Chinese celebrate the Moon Festival (a major holiday second only to Chinese New Year). Lanterns are made, moon cakes eaten, and gifts exchanged.
As the transition to fall begins, fall produce start appearing. For the U.S., generally that will be apples, artichokes, cranberries, edamame, pears, pumpkins and quinces. Pumpkin spiced beverages start appearing, but it seems some retailers jumped the shark early and started serving them in August. Hard to think of autumn in August or early September when the outside temperature might in 90 F (32 C)!