September is the ninth month of the year in the Gregorian Calendar and one of four Gregorian months with 30 days.

In Latin, septem means "seven" and septimus means "seventh"; September was in fact the seventh month of the Roman calendar until 153 BC, when there was a calendar reform from the month of the Ides of March to the Kalends, or January 1.

September begins on the same day of the week as December every year, because there are 91 days separating September and December, which is a multiple of seven (the amount of days in the week).