The original "Oktoberfest" occurred in Munich, on October 12, 1810: For the commemoration of their marriage, Crown Prince Ludwig (later King Ludwig I) and Princess Therese of Saxe-Hildburghausen (namesake of the Theresienwiese festival grounds) organized a great horse race (the marriage took place on October 12; the horse race on October 17 — therefore, there are different dates named as being the first Oktoberfest).
Well, I guess that still doesn't explain why its in September.
Saturday we hit the place around wicked thirty and started pounding Sam Adams Oktoberfest Ale. One stupid thing, we walked right past an empty ATM on 7th and then had to stand for 15 minutes in line at the one across from Fountain Square. Plan ahead people.