Sunday has always been the most popular and relaxed day for looking at art, as museum attendance readily attests. But until now, few serious commercial galleries, here or anywhere, have taken advantage of the roving bands of eager Sunday brunch-and-culture seekers by opening their own doors.

There are two longstanding exceptions: Henri Gallery at 1500 21st St. NW (where minimal "bottle" paintings by Katharine T. Carter are on view) and Capricorn Galleries at 4849 Rugby Ave. in Bethesda (now featuring realist painter Sueo Miyagawa). Capricorn's Phil Desind, who hasn't missed a Sunday afternoon in 21 years, says, "It's our best day. I don't understand why more people don't do it."

More people are doing it, notably two promising young gallery owners in Adams-Morgan: Jack Shainman (2443 18th St. NW), who says he simply opened his eyes, saw the demand and responded by opening Sundays from 2 to 6 p.m.; and Marie Martin, just a few doors away (at 2427 18th St. NW), who has done likewise.

Both share a special advantage: Their galleries are located in a genuine neighborhood that brims with weekend activity and some of the most agreeable restaurants in town. Other similarly situated dealers -- especially those around Dupont Circle -- have been watching with interest, and one of them, Addison/Ripley (9 Hillyer Ct. NW) will be open tomorrow as an experiment from noon to 5.

It makes a nifty Sunday package: a museum, a gallery or two and a pleasant meal or high tea. The following shows can all be seen on Sunday, as well as during regular hours today and on weekdays. Rediscovering John Robinson

Every time John Robinson's paintings surface in Washington, the same thing happens: People say gee whiz, gadzooks, this man is wonderful, where has he been all my life? They said it after his first one-man show (when he was 64) at the Corcoran in 1976, and they said it again after his retrospective at the Anacostia Museum in 1983.

Now they're saying it at Jack Shainman Gallery, where Robinson is once again warming hearts with paintings that tell of a sensitive, intelligent artist's life -- of his large, happy family, and of the beauty he (and few others) has always seen in the hills, houses and lilacs surrounding his home and studio in Anacostia. Even Shainman knew nothing of Robinson until he saw his work in a recent Washington Project for the Arts group show, titled "The Afro-American Presence in Washington." Thus, at 73, Robinson surely qualifies as Washington's most rediscovered artist.

The question isn't where Robinson has been, but where the art world has been all his life. And the answer is clear: It has not been in Anacostia. Robinson, in fact, has been making art since age 12, when he was "discovered" for the first time by a customer in the Georgetown garage where his grandfather was night watchman. His patron arranged for Robinson to take art classes at Howard University under James Porter in exchange for cleaning chores. That training, says Robinson, "remains the basis of my efforts in art." Thomas Hart Benton was another, conscious or not.

This show -- the first of three Shainman plans over the next three years -- is devoted to "The Early Years: Paintings 1935-65" and includes some of Robinson's most memorable works, many dealing with the reflections and self-referential echoes that so intrigue him. There is, for example, a painting of a painting, in this case a self-portrait of Robinson as a young man, complete with the frame and glass covering. The glass glitters with the reflected contents of the artist's studio.

And there is the Norman Rockwell-like portrait of Maude Jones, a woman who sold newspapers at the corner of 14th Street and New York Avenue NW and who wanted more than anything to have a portrait of herself reading the Bible. Robinson not only obliged, but also reproduced the painting within a larger painting, also on view, of an outdoor art fair in Lafayette Park. These fairs, annual events sponsored by the Washington Times-Herald in the '40s, were an ongoing source of nurturance and prize money for the artist, who (as he so gently puts it) was not welcome everywhere. He supported his family by working as a kitchen helper and later as a supervisory cook at St. Elizabeths Hospital. He also painted several church murals and backgrounds for portrait photographers downtown.

There's a good deal more to know -- and see -- of Robinson's art, but this show, continuing through Jan. 23, is a fine place to start. Shainman, by the way, is opening a second gallery in New York's East Village today. Dickson Carroll's Furniture------

For sheer, unadulterated fun, you aren't likely to beat Dickson Carroll's fanciful, carved art/furniture (including dressers, desks, mirrors and even a double bed) at Addison/Ripley Gallery, tucked in the alley behind the Phillips.

A sculptor and architect, Carroll is best known hereabouts for his cheerful, colorful and beautifully crafted wood carvings made from sensuous, biomorphic shapes-within-shapes, often topped with flamboyant Gothic flourishes and touched here and there with brightly colored enamel paint.

He has now brought all of his talent, imagination and splendid craftsmanship -- along with some newly acquired design restraint -- to bear upon the practical matter of making furniture: an ingenious front hall mail table, a stereo cabinet that has the pizazz of an antique jukebox, a pyramidal bed carved and painted to ensure happy dreams. It is absolutely his best work -- and his best show to date. But you'll have to hurry. The show closes tomorrow. Hours today are 11 to 5.

Apart from the furniture, by the way, the show also includes a model for a public sculpture that recalls the turreted bandstands, glorious gazebos and other architectural projects that Carroll proposed in his last show. None, alas, was built, though any one of them would have improved by at least 1,000 percent the appalling mess of so-called "public art" at the Bethesda Metro station. If you know a builder or an architect, drag him to this show, which comes just in time to complement furniture-maker Wendell Castle's exhibition of clocks at the Renwick. If Carroll's furniture requires from its owners a bit more daring, abandon and humor (and less money) than Castle's, it is highly livable furniture of the sort that's bound to make its owners smile whenever they see it. Timothy Lamb's Photographs

Timothy Lamb's hand-worked Cibachrome color photographs at Marie Martin Gallery are wholly unresolved, his point and purposes unclear. Gigantic in scale, they attempt to look like the paintings of Francis Bacon by featuring blurred images of people with a bizarre tilt, heightened by the addition of various dyes. The "Wall Street" series is better, but not by much.

But Martin has a good deal more to show visitors in her handsome new space, including vintage photographs by Ansel Adams, Robert Frank, Walker Evans and other photographic masters (and good books on photography as well), along with patterned cutout sculpture in basswood by Renee Balfour and pit-fired and painted ceramic sculpture by Darrell Dean.