From The Art and Popular Culture Encyclopedia
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 number of days in the week). No other month ends on the same day of the week as September in any year.
In Latin, septem means "seven" and septimus means "seventh"; September was in fact the seventh month of the Roman calendar until 46 BC, when the first month changed from Kalendas Martius (1 March) to Kalendas Januarius (1 January). It is the sixth month of the Astrological calendar, which begins end of March/Mars/Aries.