There is only one big event to the musical week. But it is big in more ways than one.

On the Fourth of July, Leonard Bernstein will conduct the National Symphony on the West Terrace of the Capitol. It is the first time Bernstein has taken on this symbolic assignment. The central music will be his song cycle, "Songfest," a work he wrote for the American Bicentennial, based on the works of a wide variety of American poets.

And, as if that were not enough, he will conduct his symphonic dances from "West Side Story." DANCE

Nightly Dance Parties continue at the Smithsonian's Festival of American Folklife tonight, and resume Wednesday through Sunday except July 4; the free events feature dance demonstrations and participation led by groups from New Orleans, rural Louisiana and Wisconsin.

The recently established Kankouran West African Dance Company presents a program titled "A Visit to Africa" Wednesday night at the Marvin Center Theatre. FILM

The American Film Institute kicks off its "Rock, Roll, and Culture" series this Saturday with "Jazz on a Summer's Day," a documentary about the 1958 Newport Jazz Festival featuring Chuck Berry, Louis Armstrong and Thelonious Monk, among others; Saturday and Sunday, "A Hard Day's Night" in a double bill with "Help!" Call 785-4601 for details.

Monday through Wednesday at the Sidwell Cinema, "Repo Man," with fine performances by Emilio Estevez and Harry Dean Stanton. Call 537-8178 for details.

The National Theatre's Monday night series continues with Jimmy Cagney in "Yankee Doodle Dandy." Call 783-3372 for details and reservations.

Among current releases, John Huston's "Prizzi's Honor," starring Jack Nicholson. POP MUSIC It was 20 years ago today, Sergeant . . . whoops, wrong band. Right amount of time for the Grateful Dead, though. They'll celebrate the beginning of their third decade with shows at the Merriweather Post Pavilion tonight and Monday.

Singer-songwriter John Martyn, whose jazz-influenced folk-rock has long been well received in Europe, makes one more foray into America, on Monday at Club Saba.

Chick Corea in a club: that's a treat, as the pianist brings a trio to Blues Alley on Tuesday and Wednesday. And, starting an eight-day run on Friday is singer Tony Bennett, who seldom plays the clubs, either.

The Sixth Annual D.C. Loft Jazz Festival at d.c. space kicks off on Wednesday with the Windmill Saxophone Quartet (9 p.m.) and the Malachi Thompson Freebop Band (10:30 p.m.), followed by the Steve Williams Ensemble (Thursday, 9:30 p.m.); the Ron Holloway Quintet (Friday at 10) and the East-West Quartet (Friday at 11:30); concluding on Saturday with the Webster Young Quintet (10 p.m.) and the Trio (Hayes Burnett, Nasar Abadey and Jimmy Lyons, at 11:30 p.m.). It's a great opportunity to discover some local jazzmen whose work is decidedly adventurous. THEATER

"The Foreigner," the sweetest, silliest comedy to come along in months, gets Olney Theatre's season off to a smashing start. In a Georgia fishing lodge, a pathologically timid Englishman tries to pass himself off as a foreigner incapable of comprehending English.

How he is lured out of his shell -- and defeats a rampaging band of Ku Klux Klanners in the process -- makes for off-the-wall hilarity. It's hard to imagine anyone who will give a more endearing performance this summer than Patrick Richwood as a backwoods nitwit who surprises himself and the others by acquiring a few brains.