Why is the Oktoberfest called the "Oktober-Fest" when it starts in September?
This has something to do with the Munich weather at this time of the year, which is much nicer and warmer in September than October. In this manner, guests can enjoy strolling around the fair grounds or sitting at the adjoining areas outside the tents without freezing.
The reason it starts in September has also historical origins. At the beginning of the 19th century, when the fair was still in its infancy, it went for a much shorter period of time, and only took place during October (ending on the first weekend of October). This consequently made it an October-fest. Further historical information regarding the Oktoberfest can be found here. (Please go to the link above)