Against a backdrop of winking Christmas lights and a stage littered with "go-go babies," a troupe of government workers known as The Sunshine Skiffle Band brought Dixieland to the mezzanine of the National Theatre this week.

To the musical cacophony created by clattering washboards, gourds, kazoos, a Jamaican rumba box and traditional jazz instruments, about 120 persons lounged on the theater's stairways and chairs to enjoy the opening of the 1981 Monday Night at the National concert series.

The crowd of after-work revelers laughed, slapped their thighs and sang the night away to the band's lively renditions of turn-of-the-century jazz tunes such as "Coney Island Washboard Rag" and "Yes Sir, That's My Baby."

Babies crawled contentedly on the floor and children danced on stage, giving the concert the appearance of a late-night romper room. As the high-pitched wail of the clarinet swelled, ghostly visions of flappers and jazz-age dandies danced around the room.

It wasn't grand theater -- just grand fun.

"It's such a good time it's infectious," declared Tom Moriarity, a newcomer to the city who spent the evening flashing an ear-to-ear grin. "I came after work and I'm coming back."

The free performances, scheduled for every Monday at 6 and 8 p.m., were first offered by the National last October "to open its doors to the neighborhood," said Kathy Barry, public-relations director for the theater.

"And it really has. I thought we'd get a lot of college students, but the first night (the 1979 series was offered) the women with the blue hair and heels turned up. Then the people with babies," Barry said. "People are coming on the subways, after work, off the street -- all because they heard about it from someone. It's wonderful."

Barbara and Evan Arrindell think so. They had bundled their active, 7-year-old son Dean into the family car and driven from Gaithersburg for the night out on the town.

"I enjoyed it. If we didn't go out this way, we'd spend our whole lives at work and at home," said Barbara, who added that she scans the newspapers in search of free family entertainment, to offset the high cost of living.

The evening family activities also give Washington a "cosmopolitan" flavor that draws suburbanites into the city, her husband added.

"Everything in the suburbs closes down at six o'clock. Things like this get people into the city," he said.

In weeks to come performers will include: (Jan. 12) Deco, a de-wop trio that sings oldies and top-40 tunes; (Jan. 19) The Victorial Lyric Opera Company, and (Jan. 26) The Foggy Bottom Morris Men, a lusty band of singers and dancers performing ancient dances.

Barry said the performers, who volunteer their services, are selected for their audience appeal. She said The Sunshine Skiffle Band, for instance, has become a favorite at area folk festivals.

Gil Carter, a D.C. attorney and apparent leader of the group, said the band of jazz musicologists and Dixieland enthusiasts was formed three years ago. The group is composed of washboard players Beale Riddle and "Washboard" Mike, John Jenkins on alto sax, Don Rouse on clarinet, Dave Littlefield on banjo and Dave Robinson on coronet. Carter plays the rumba box and sings.

The group's appeal lies in its ability to give an energetic, original sound to some very old music, all the while evoking a spirit of fun.

"I know all these songs. They keep alive music that would otherwise be dead," said Malcolm Oettinger, whose eyes were bright as a child's as he listened to the music he said he collected as a boy on 78-rpm records.

"I came straight from work to hear them. I was delighted it was free, but i would gladly have paid."