Leave it to the Germans to have a party so epic, they have been celebrating it for 180 years now.
In America, Oktoberfest has become an excuse for people (and by people I mean me) to get black out drunk off liters of beer and loudly yell lewdly inappropriate things in Deutsch. Oktoberfest has become to the average consumer what Easter and Christmas are to church goers; it's the one day of the year they indulge in their favored, yet infrequent activity.
Our bastardization of the Bavarian holiday has been digested, marketed, and truncated by America and our Bar-not-a-Pub culture. Thanks to our heavy German brewing past, the event has persisted, but if you asked the average too drunk forty year old at the local Oktoberfest celebration, they will usually have no idea that it started in Bavaria because of an 1810 wedding between King Ludwig I and Princess Therese who invited everyone. Literally everyone. And it was a reception that lasted 16 days and ended with horse races. Did you have horse races at your wedding? Yeah, didn't think so.
Everyone had so much fun with the parade, and races they said, "Fuck, lässt es wieder tun! (lets do it again!)" And so they did, and there was much rejoicing.
The agricultural festival that was added and the parade are the few things that have lasted from the first events until now. It has evolved over the years and the things we see as iconic were picked up as the years went by. Beer wasn't served in the glass mugs we associate with the festival til 1892, the horse races were dropped in 1960, and the food has changed with the German palate.
Oktoberfest, or "die Weisen," in so entrenched in German culture that even the bigger socio-political environment impacts the fest. Days were changed so that October 3rd, German unification day, could be celebrated. There have been 24 cancellations of the fest due to cholera and war.
The American celebration of Oktoberfest has become the German St. Patty's day. We get drunk, we eat sausage and sauerkraut, and wear unfortunate liederhosen. What has become a casual drinking holiday for us, is a 17 day marathon of fun to Europeans and an event that is steeped in the historical evolution of its host country.
When you're chugging your liter of beer, just remember to pour one our for good ol Ludwig. If he hadn't had the most epic party ever, you wouldn't be a "Beirleichen" or beer corpse these next two weekends.