This is a strange and remarkable strip, and not just because Snoopy's using full thought balloons for either the first or second time now. (I think I remember a prior use of the bubble-tailed balloons, in a single strip. But up until now all of Snoopy's other thought bubbles have had tapering tails like speech bubbles.)
The hopak is a traditional Ukrainian dance. The MIGHTY PEDE says it is sometimes known as the "Cossack Dance." In the United States we tend not to have traditional national entertainments of that sort (except, of course, for terrible action movies, brainless reality shows, sports team blathering and Fox News). Anyway, Snoopy really sells this one, and other than for the folded paws this becomes what we might identify in the middle period of the strip as the Snoopy dance. I think we've seen him do it once before, but here it is identified as a dance.
Schulz probably chose a Hopak because it's entertaining to see a dog perform it, and to draw Snoopy doing it, and it's an especially nice trick for one, but it's still conceivable unlike, say, a waltz.
Most comic strips subtly change art styles through the years. The Peanuts characters change a fair bit, but most characters are recognizable in their later forms. Snoopy pushes this the most; he's much changed in these early strips and the furred, bipedal, typewriter-using, figure-skating, Sopwith-flying, moon-landing creature of the later years.
The Snoopy Dance is relevant to this because its primary identifying characteristics are the upright posture and the flapping hind legs. Both are no longer unusual in Snoopy's late bipedal stage. Perhaps recognizing that, Snoopy's dance moves become a more general, smiling prance rather than a modified Hopak, which is a shame.
Moving on to the other characters, they are quite lively in this one, with everyone clapping and shouting "Hey!" I think this is the best party atmosphere we've seen in the strip to this point. It's also another ensemble strip without Shermy, that loner.
Isn't that rather a lot of food Violet is giving to Snoopy? I don't mean for a dog, I mean for anyone.
