At Oktoberfest most brewers traditionally have sold a Märzen style lager. To understand why, you have to go back to 1553.
There are two theories as to why in that year Albert V, Duke of Bavaria decreed that Münchners could not brew beer during the period between St George's Day (April 23rd) and St. Michael's Day (September 29th).
The first is that brewing requires a lot of heat. In the close confines of the already large town of Munich, that meant a lot of fires were needed to quench the Bier-thirsty city. Apparently Albert was tired of entire blocks of his capital going up in flames.
The second theory is that brewers were less able to control fermentation during warmer weather. It wasn't until Louis Pasteur's microscope more than 300 years later saw that Bier contained living organisms that brewers ever knew that there was such a thing as yeast. Therefore, they obviously didn't understand how other microorganisms affected Bier and its taste. Not understanding the science behind bad Bier, the Duke just forbade new Bier until fall.
Whatever the reason, this created a conundrum for the city: How do you brew enough Bier to last all summer long? The answer was to brew it at a higher alcohol level and to add more hops so as to preserve the Bier for up to six months. The usually darker style was known as Märzen, named for the month of March (März in German) when most of the summer Bier was brewed.
And so this brings us to Oktoberfest. Is it just the world's greatest Bier drinking marketing ploy ever devised so as to clean out the cellars of old inventory in order to make way or a new season of Bier? Whatever the reason, we like it; so just raise your Stein and . . .