Oktoberfest dates back to 1810 when Prince Ludwig, later Ludwig I, married Princess Therese. The celebration initially was a sixteen-day celebration of the wedding. It became an annual event starting in September, and ending on the first Sunday of October. Later, to include German Unity Day, which is October 3, if the first Sunday of October is before October 3 the celebration is extended to end on German Unity day, October 3. So, despite its name, it is more in September than October.
While the main celebration is in Munich, Germany, and it is a Bavarian celebration, there are celebrations elsewhere.