This is effectively just another episode for the first season and there is no real reason not to watch it. Watching this after the first season makes sense. The story is about the k-on girls performing at a small concert with other bands. The pacing was faster than normal, maybe a bit rushed, and I didn't like some scenes but it was not a bad episode. The characters, animation and voice acting are all the same as season 1; there's just a few different characters in the form of the other band girls. Overall it's another k-on episode, watch it if you're watching (and enjoying)the anime, don't watch it if you're not.