The history of September (Summary)

The word September was derived from "septem", that means seven in Latin, but September is the ninth month of calendar. The reason is because September was the seventh month in the Old Roman calendar, that started in March, when the Romans was living under the rules of Julio Cesar. After the Julian calendar reform, our calendar was shifted by two months, starting now in January, 1. This way, September became the ninth month of the calendar, and one of only four months with 30 days.

Original: http://blog.dictionary.com/september/