The Washington Post critics choose their favorite shows of the week. ART

The extraordinary Chagall retrospective exhibition at the Philadelphia Museum of Art continues to break attendance records there, and the museum has decided to extend the show's closing date to July 21. It is a memorable show that allows the viewer to relive the excitement early in the century when Chagall arrived on the scene, and to immerse oneself in images often taken for granted nowadays -- the fiddlers, flowers, lovers, animals, angels, synagogues and cityscapes that populated his universe. CLASSICAL MUSIC

Summer is here, but this week's classical music agenda is one of the year's busiest.

There are two major week-long events, both opening Tuesday.

At the Kennedy Center, there is the Mostly Mozart Festival, sure to be of special appeal in this Year of "Amadeus." It is an annual preview of events to be included in Lincoln Center's Mostly Mozart Festival, and will stretch over five successive nights -- each with different programs and soloists. Just as an example, opening night will be under the festival's music director, Gerard Schwarz; 17-year-old Joshua Bell will make his Kennedy Center debut in the Third Violin Concerto, K. 216. And Horacio Gutierrez will perform in the most heart-rending of the piano concertos, No. 24 in C-minor, K. 491.

Meanwhile, the University of Maryland will play host to the Eighth World Saxophone Congress, at which hundreds of performers and ensembles from a multitude of countries will play in virtual nonstop events until the closing concert on Saturday. Certainly none of its events will be more spectacular than the alfresco performance by all these saxophonists at the Capitol on Thursday at 4:30 p.m.

The National Symphony also begins its Beethoven Festival at Wolf Trap on Thursday with pianist Bella Davidovich and conductor Hugh Wolff. Saturday night pianist Andre-Michel Schub will play, also under Wolff. DANCE

Nine area dance groups, representing classical ballet, modern, tap, breaking, folk and national ethnic dance cultures, will perform free this afternoon as part of the Kennedy Center's first Open House. Free nightly Dance Parties, featuring dance demonstrations and participation of a New Orleans blues band, a Louisiana Cajun group, and a Finnish-American polka ensemble from Wisconsin, will be among the attractions on the Mall this week, starting Wednesday, in the Festival of American Folklife. FILM

The American Film Institute continues its Five European Directors series with films of Truffaut, Leone, Wajda, Herzog and Larissa Shepitko.

The American Screenwriters series at the Library of Congress concludes Wednesday with "I Wanna Hold Your Hand," scripted by Robert Zemeckis and Bob Gale, at the Mary Pickford Theater, 7:30.

On Thursday, the Black Music on Film series at the University of the District of Columbia continues with "Princess Tam Tam" starring the talented expatriate dancer and music hall star, Josephine Baker.

Among current releases, John Huston's mordantly funny "Prizzi's Honor," with a hilarious performance by Jack Nicholson as a dumb but romantic hit man. POP MUSIC

At Charlie's, it's goodbye to all that, as the intimate Georgetown jazz club goes into its last week with the man who gave it a name and many a good night, guitarist Charlie Byrd. Next Sunday is the last night.

Nostalgia triumphs this week: for old folkies, it's Tom Paxton, the Kingston Trio and the Smothers Brothers at Wolf Trap on Sunday; for R & B buffs, it's the Johnny Otis Show at Charlie's on Monday and Tuesday.

Black Flag, one of the top hard-core bands in America, and fronted by Washingtonian Henry Rollins, enlivens the 9:30 club on Tuesday and Wednesday.

Twenty years ago, the Last Poets suggested foundations for rap; revived, they appear at Kilimanjaro's on Wednesday.

Howard Jones knocked 'em dead at the Bayou last year; now he's on the verge of letting the world know that synthesizers can be human. At the Post Pavilion on Wednesday.

Accordionist Stanley (Buckwheat) Dural wore out everybody's dancing shoes last month at Friendship Station; he and his irrepressible zydeco band have a return engagement on Wednesday.

Lone Justice, and lead singer Maria McKee in particular, have been hailed as the next Quarterflash; in fact, their country-punk synthesis is a lot more interesting. They make their Washington debut at the Bayou on Thursday.

Toure Kunde, led by three Senagalese brothers, are the hottest African Pop group to hit the states since King Sunny Ade; they'll be joined Saturday at the Warner by Brazilian star Alceu Valvenca. 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 will give a more endearing performance this summer than Patrick Richwood, as a backwoods nitwit who surprises himself and others by acquiring a few brains.