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This month's question: What is the difference between Märzen and Oktoberfest? How would I make one?

Answer: The BJCP lumps these two beers together into a single style, number 3B in the 2004 version of the BJCP guidelines. It is found along with its sister beer, Vienna lager, in European Amber Lager.
If you know even the slightest bit of German, you probably know that Märzen is the German word for March. Even if you don't know any German, you can probably figure out which month Oktoberfest is named for. Shortly after the invention of lager in the early 1800's, but before the advent of artificial refrigeration, the German, Austrian and Czech brewmasters discovered that their beer tasted better when brewed in cool months. The last batch of the spring season they called Märzen. Later in the fall, they pulled some out of storage for the "fest" celebrations. Generally, these fall beers were brewed a bit stronger, so they would last through the summer months better. Eventually, with the advent of refrigeration, the need for seasonality disappeared, but traditions prevailed. So, if you want to get technical, you would call it Oktoberfest if the starting gravity is at the high end of the range, and Märzen at the low end.

The backbone of any Märzen/Oktoberfest beer is Vienna malt. Ideally, it should be made from Moravian barley, the same grain used for Pilsner malt. Slightly different roasting conditions lead to a slightly darker color and richer malty taste. Often, various amounts of Pilsner malt and Munich malt are also added. Crystal malts can be added too, though these should be added sparingly. The flavor should emphasize melanoidin character, not caramelly character. Work out the total grain bill to hit a gravity of 1.050 for Märzen, and bump it up to 1.056 for Oktoberfest.

Märzen/Oktoberfest is not a strongly hopped style, so go easy on the hop additions. Traditional noble varieties are used, with Styrian Golding and Saaz being the two most authentic. Keep the bitterness between 20 and 28 IBU. There should be no hop aroma, though a bit is allowed in Vienna lager. Hop flavor is low also, so keep most of the hop additions early in the boil.

The preferred process for all-grain brewers is to use decoction mashing. This will enhance the malty melanoidin flavors. However, if you want to skip the extra effort, just bump up the Munich malt portion of the grain bill a bit, and use a step infusion mash. For you extract brewers, get some Munich malt extract and mix it 50:50 with pale malt extract. Add a bit of steeped low-Lovibond German crystal malt you should be good to go.

Water with moderate carbonate levels should be used. This is in contrast with Bohemian Pilsner beers, which use very soft water. The style requires good clarity, so add some Irish Moss of Whirfloc to the batch near the end of the boil.

These are lager beers, so make sure you have your lager fridge ready. Pitch with a yeast such as Wyeast 2206 or White Labs WLP820. Ferment between 48°F and 56°F. For Märzen, go about 8 to 12 days. Then, transfer into secondary and lager the beer for at least 4 weeks in the lower 30's. For an authentic Oktoberfest, rack the primary a bit early, when the beer's gravity is about 1.017 or so. Now comes the hard part: resisting the temptation of drinking your beer before the usual lagering time of 6 months. These beers benefit from kräusening, though the usual methods of adding priming sugar or force carbonation work well.

Vital Statistics for Vienna and Märzen/Oktoberfest