The outdoor festival season begins in earnest this weekend with dueling Oktoberfests. Das Best Oktoberfest unfolds this Saturday and Sunday at National Harbor. The tentative drinks menu lists more than 60 beers, including a few newcomers like the limited-edition Gordon Biersch Imperial Pilsner Brau and Duckpin Pale Ale as well as the Balt Altbier from the recently opened Union Craft Brewing Co.