Bavarian Crown Prince Louis, later King Louis I of Bavaria, marries Princess Therese von Sachsen-Hildburghausen. The Bavarian royalty invited the citizens of Munich to attend the festivities, held on the fields in front of the city gates. These famous public fields were named Theresienwiese—"Therese's fields"—in honor of the crown princess; although locals have since abbreviated the name simply to the "Wies'n." Horse races in the presence of the royal family concluded the popular event, celebrated in varying forms all across Bavaria.
The decision to repeat the festivities and the horse races in the subsequent year gave rise to the tradition of the annual Oktoberfest, which now begins in late September and lasts until the first Sunday in October. Alcohol consumption is an important part of the modern festival, and more than 1 million gallons of beer are consumed annually at Oktoberfest.
Taken from: http://www.history.com/this-day-in-history/the-origin-of-oktoberfest [12.10.2012]