September is the ninth month on the Gregorian and Julian calendars. On the old Roman calendar, September was the seventh month since their calendar started in March. When the calendar was revised to add January and February, the name Septem (meaning seven) remained. In the Northern Hemisphere, September is when autumn begins but in the southern half it is the first month of spring.

September is traditionally the start of the academic year for most schools, though some do start in late August. The Eastern Orthodox Church starts its liturgical year in September. The Autumnal Equinox takes place between September 22-24 and marks astronomically the first day of autumn. The first full moon of September is often called a Harvest Moon since many farmers begin harvesting crops that are harvested in the fall.

While daytime temperatures can remain warm during September, generally the nights start getting cooler and the sun starts setting much earlier. And generally, the sun starts come up later in the morning resulting in people going to work when it is still dark. The birthstone for September is the sapphire (represents clear thinking) The September flowers are the forget-me-not, morning glory, and the aster.