Arcade

The story takes place between episodes 20 and 21 of the second season, after Ritsu takes Mugi to the game center a few episodes before (which is mentioned a lot). The winter days episode in Season 1 is brought up, in which Mugi has a job (you'll find out why later). Okay, so maybe Japan's arcades are different from ours, but the K-On girls in an American-style arcade? Awesome!