Near Liberty

New York may have a corner on the Statue of Liberty hoopla, but Washington has a couple of activities of its own:

*Along with members of Congress and probably President Reagan, you too can sign the world's largest birthday card, in honor of the Statue of Liberty. The card, which measures 135 by 95 feet, will be at the Washington Convention Center Friday from 11 to 8; Saturday 9 to 6. Donation is $1 per signature (this includes use of a pen). The card is at the H street entrance of the convention center at 900 Ninth Street NW. It will hang in public view from July 2-6 at the Washington Building (1440 New York Avenue NW). 682-1400.

*Meridian House International displays one of six original models of Liberty cast in 1886 by Frederic- Auguste Bartholdi, the French creator of the harbor statue, as part of Liberty and Enlightening the World. The little lady stands 50 inches tall (on a three-foot pedestal) until August 15. She's yours, incidentally, for a cool $250,000. Meridian House (1630 Crescent Place NW) is open weekdays and Sunday 1 to 4 (closed Saturday). Free. 667-6800. From the Ground Up

Architecture buffs should head on down to the National Building Museum from noon to 4 Saturday for Celebrating Building Ornament, a free program and exhibit examining the intricacies of building buildings. Eleven craftsmen demonstrate the art of brickwork, ironwork (scrolling wrought iron), terra cotta tile making, plasterwork, woodwork and crimping sheet metal. You can take 20-minute tours to learn how the Pension Building manifests such craftwork at 12:15, 1:15, 2:15 and 3:15. Craftsman John Barianos shows off the gold leaf and patina skills that he used on the U.S. Capitol, Union Station and the Willard Hotel. There'll also be an area for children's activities where kids can make tiles of their own. The museum is in the Pension Building, F and Fifth streets NW (at Judiciary Square). 272-2448. Mozart to the Max

Long before Amadeus made it big in Hollywood, the Kennedy Center was already giving old Wolfie his due with the Mostly Mozart Festival, which continues this weekend in the Concert Hall. In addition to pieces by Mozart, the Mostly Mozart Orchestra plays music from Haydn, featuring pianist Richard Goode and violinist Cho-Liang Lin on Friday. Saturday's program includes music from Mendelssohn and Schubert; guest artists are John Browning on piano and violinist Young-Uck Kim. Tickets for both shows, which begin at 8:30, are $13.50 and available at the KC Box Office, 254-3600 or 857-0900. The soloists play a pre-concert recital each night beginning at 7:30. Dancing on the River Terrace to a three-piece combo winds up each evening beginning at 10:30. Source of Fun

The Source Theater company gets into the carnival business with A Midsummer Day's Dream, a benefit event this Saturday from 10 to 4:30. Among the free entertainment are scenes from "Cowboy Mouth," "Narcissus Bound," "Skyfall" and "Beyond Therapy," all Source productions. Other entertainment includes live dance, mime and music from such artists as Pete Kennedy and Mike Vargas. (Source founder and artistic producer Bart Whiteman makes his country-and-western debut). There'll also be an auction and food and carnival booths. The day ends with a performance of the comedy, "Hitchhiker's Guide to the Galaxy" at 5. Tickets for the play are $5. The event is in the alley behind the Warehouse Rep Theater, 1835 14th Street NW. 462-1073. A Big Gig

The American Film Institute in the Kennedy Center presents the Washington premiere of The Gig, a dramatic comedy about six amateur jazz artists who find their inspiration when they play at a Catskills resort. The film stars Wayne Rogers (Trapper John McIntyre of TV's "M*A*S*H") and Cleavon Little ("Blazing Saddles"). Director Frank D. Gilroy will be there to discuss the 8:15 screening. Tickets, $4; $3 for AFI members, are available at the door. 785-4601. Rest of the Best IN TOWN

Connecticut Connection's City Arts Series opens Friday in a free noon concert with the Art Monroe Quartet and the Junkyard Band on the outdoor arcade at Connecticut Avenue and L Street NW. 783-1101 . . . From noon to 8 Saturday, the Centro de Arte Community Cultural Center, 15th and Irving streets NW, hosts the 3rd Annual New Song Festival, a benefit offering ethnic music from such groups as Rumisonko and the South African Students Choir. Ethnic food will also be available. Tickets, $3, are available at the door. 483-5825 . . . 17 Dupont Circle fine art galleries band together for the 2nd Annual New Talent Open House Saturday from 11 to 5. The galleries feature exhibits by local artists. Maps are available in the Phillips Collection bookstore or at the participating galleries. (They're within a four-block area between O & R streets/19th and 22nd streets NW; look for balloon displays out front.) Admission is free. 328-0955 or 223-6683. IN MARYLAND

The National Christian Choir, made up of 200 voices -- including those of Wintley Phipps and Sue Dodge -- from 70 area churches, boom patriotic tunes in the concert, "Blessed is the Nation" Saturday at 4 and 8 at Thomas S. Wootton High School, 2100 West Ritchie Parkway, Rockville. Tickets, $6 and $8, are available at the door. 938-8587.