Oktoberfest is the German celebration of beer. In the days before refrigeration, Germans drank beer only in the cool weather months. Thus, Oktoberfest was the celebration of the availability of the first batch of beer after a long, dry summer. It is two weeks long, ending the first weekend in October (thus, most of Oktoberfest occurs in September).

The end of September is a common time for various beer celebrations. Here is a good one:

Texas Craft Brewers Festival
An Outdoor Beer Sampling Festival Dedicated to the Fine Art of Craft Brewing
September 24, 2011
2 – 8pm
Fiesta Gardens, Austin, TX
http://www.texascraftbrewersfestival.org/