The weekend’s best in nightlife, music and art. For even more, check out Nightlife Agenda.

EL-P (left) and Killer Mike will team up to perform "Run the Jewels" at the 9:30 club on Saturday. While they may still need an introduction, Baltimore gets a visit from a duo that doesn't: Jay-Z and Justin Timberlake. (Marlon Correa/The Washington Post)

Friday: Summer concerts at federal parks took a hit this summer, but the free Reggae Night returns to Carter Barron Amphitheatre, featuring local bands the Archives, Image Band and Strykers Posse. Gates open at 7 p.m. to the first 3,700 people; the show begins at 7:30 (come early and picnic in the park outside the amphitheater). No advance tickets are required, but there's no rain date.

Friday:   Story League's storytellers are far more likely to share their deepest, most regrettable moments than to show off their singing chops, but on occasion, like all of us, they're moved to break into song. Friday, the storytelling show returns to the Black Cat‘s mainstage for Story League Sings, a story-and-karaoke show centered around the theme of “Bad Girls,”  with accompaniment by the HariKaraoke band.

Friday: It seems like 3 Stars has been around for more than a year, but the prolific D.C. brewery is celebrating its first birthday Friday at ChurchKey. Owners Dave Coleman and Mike McGarvey are bringing 20 beers that show off the wide variety of flavors they create, from potent Coffee Pandemic (a hoppy imperial porter brewed with cold-pressed Yirgacheffe coffee) to the refreshing, summery Lime Basil Saison. Expect the celebration to run all night.

Friday-Aug. 17: The Montgomery County Agricultural Fair has rides and a demolition derby and petting zoos and a monster truck rally, but the best thing about spending a summer day on the midway is that society won't judge you for eating anything (everything?) that's deep-fried. If it can be submerged in hot oil, it can probably be found at the Montgomery County Agricultural Fair, which opens at 3 p.m. Friday and runs from 10 a.m. to midnight through Aug. 17.

Saturday: The Ridiculous Obstacle Challenge 5K is much closer to an episode of "Most Extreme Elimination Challenge" than any other 3.11 mile race you've run before. Lace up your running shoes and prepare for a course that includes a wrecking ball, moon bounce, rope swings and an inflatable slide. Don't worry about posting a personal best: The event, which is open to anyone 13 and older, is untimed. And frankly, when else are you going to get a little time on a moon bounce?

Saturday:  Rapper Killer Mike and producer/rapper El-P have forged a unique rap partnership that could be compared to that of Jay Z and Kanye West. On a perfectly composed track, it's nearly impossible to pick apart the producer's contribution from the rapper's. On last year's "R.A.P. Music," Killer Mike's rhymes were so tightly wound around El-P's hard-driving, electronically charged beats that it made sense when the pair put both of their names on this summer's excellent and menacing "Run the Jewels." This time, El-P also raps.

Sunday:  The costumes displayed in the National Gallery of Art's ongoing "Diaghilev and the Ballets Russes" exhibition are so arresting that, at times, it's as if the movements of the dancers wearing them become secondary. As a counterpoint, this weekend at the museum,  Dana Tai Soon Burgess Company will dance "Revenant Energy," a new, 30-minute performance inspired by George Balanchine's "Le Bal," which was performed by the Ballets Russes in 1929. See the former free at 1 and 3:30 p.m. near the entrance to the exhibition Sunday.

Sunday: No blues musician is more worthy of the title "living legend" than B.B. King. The 87-year-old Mississippi native became a Kennedy Center honoree in 1995, but Wolf Trap has long served as his base of operations in the Washington region. King, who is touring as part of Peter Frampton's "Guitar Circus," has performed at the Filene Center 22 times since its 1971 inaugural season, always accompanied by Lucille, his signature, soulfully eloquent Gibson guitar. Small wonder crowds invariably greet the iconic pair with the first of several standing ovations. With Peter Frampton.