Two of the season’s biggest Broadway shows are opening tonight — just not on Broadway. The new Mel Brooks musical, Young Frankenstein, is opening in Seattle, where it is having its pre-Broadway tryout. Meanwhile in Denver, Disney’s The Little Mermaid opens this evening as well. Out-of-town tryouts are common for Broadway-bound musicals as a way to fine tune the production before getting within earshot of merciless New York theatergoers. Frankenstein and Mermaid will have their official Broadway openings in November and December, respectively.