The Monday-night crowd at JR.’s Bar & Grill (1519 17th St., NW; 202-328-0090) is unusually welcoming. “Popcorn?” a man waiting for his drink asks me. But there’s no time to eat: John Travolta has just taken the floor—or rather, the ten or so TV screens scattered around the Dupont Circle bar. The crowd—mostly gay men—merrily sings along to “Greased Lightning” from Grease. No one does the hand motions, but then again, it is only 9:30. Give it time.

Monday is show-tunes night at JR.’s, meaning the VJ plays everything from Broadway clips to the opening credits of Green Acres while music—and musical—lovers in their twenties, thirties, and forties belt out lyrics with more heart than Eddie Albert himself. The songs start at 9, the real fun at about 10:30. By then, the bar—one long room with a small loft—is comfortably packed. Soon after, you’re getting hoarse and bonding with the table next to you over your love of Zac Efron in Hairspray. Younger singers have to sit through more than a few unfamiliar ’80s clips, and the older crowd may not recognize Mr. Efron, but songs from Aladdin, Rent, and Sister Act bridge the generation gap almost scarily well.

The crowd starts to clear at midnight—it is a Monday—but the bar, if anything, gets louder. At 1:05, when the last note of “Nowadays” from Chicago falls away and an emo music video takes over, it’s almost like Cinderella’s carriage turning back into a pumpkin. (“Bibbidi-Bobbidi-Boo,” unfortunately, does not make the VJ’s playlist.)

The best part of JR.’s, though, may be watching others around you let loose. People are just happy. Well, at least most of them. There is a moment when, as Julie Andrews belts her famous Sound of Music intro, a man lets out a horrified gasp: “Oh, no! I forgot to tape The Hills!”

Hope he can get it off iTunes.