Dance Quarterly is a magazine which Squidward has a subscription to. As it says from its title, the magazine is obviously about dancing.
It is first seen and mentioned in Can You Spare a Dime?, when Mr. Krabs says to Squidward: "Hmmm, are you prepared to say that with your hand on top of a stack of interpretive Dance Quarterlies?".
It is also seen in the episode: The Camping Episode, where Squidward looks at his Dance Quarterly Magazine calendar in his bedroom about the day SpongeBob and Patrick go camping.
Dance Quarterly has a purple cover which reads "Dance" in capitalized yellow letters and "Quarterly" in lowercase white letters.