I caught this for about two minutes last night and have been unsuccessfully scouring the internet to find the episode title. Diane forces everyone in the bar to watch the Opera "Der Ring des Nibelungen" by Wagner (The Ring Cycle) and they all pretend to fall asleep. I know there's an episode where they actually go to the opera and fall asleep, but this one takes place in the bar. Please help!

That is from the teaser opening to "Showdown, Part 1" from the 1st season.

They actually attend the opera in "Diane Chambers Day" from season 4.