Monica Griffin, 14, polished off the yuletide yesterday by jostling through the shopping crowds, determined to exchange her light blue and yellow sweater for one in dark blue and red, as well as finding a different blouse and a new set of earrings.

"I'm returning all the stuff my mom bought," she explained. "She never understands what I like. I guess because she was young a long time ago."

Throughout the Washington area, the scene was much the same as the Christmas spirit of buying surged to a caroling pitch. Hundreds of thousands of shoppers hunted for bargains, exchanged unwanted gifts and made the first stab at next year's offerings.

From underwear that was too skimpy and blouses that were too baggy, to records that would make any healthy teen-ager cringe, a wide range of Christmas gifts made a full cycle -- from the giver, to the getter and back to the store.

The post-Christmas rush was largely anticipated by stores and restaurants in the area and was fueled in part by out-of-town family visitors and the estimated 30 to 50 percent of the 350,000 federal workers in the Washington area who used annual leave for a holiday.

The largest local shopping centers reported a flood of customers that rivaled any of the pre-Christmas shopping days. Parking lots that could accommodate more than 6,000 cars looked like enormous used-car lots.

The excursions yesterday, at shopping malls and downtown tourist sights, were carried off with joviality, although traffic around some of the malls was as thick as eggnog. "It's craziness, it's great," said Alice Fastov, inadvertently poking other bargain hounds with the four wrapping paper rolls, two gifts boxes and three packages of bows she coddled without much balance at Bloomingdale's in White Flint. "It's a tradition. I know the day after Christmas this is what I'm going to do and I look forward to it."

The biggest sellers were Christmas wares such as wrapping, ornaments and cards to be stored until 1985. "They just swoop it up in their arms and it never comes back," said Andy Carlton, stationery department manager at Bloomingdale's at White Flint as he loaded rolls of ribbons onto the shelf only to have them snatched off moments later.

Extra sales crews were on hand to accommodate customers. "I worked 82 hours this last week," said Tysons Corner Center record store manager John Lee as he hastily scribbled out a credit card receipt. "The first couple of days after Christmas are a little hectic, but you just get organized and you reprogram your help."

While Lee could count on five extra workers that day, most federal agencies got by with slightly more employes than on Christmas Eve, when the work force dipped by an estimated 50 percent in some departments. Jim Delisio's milk bill told the story of how light the federal employe turnout continued to be yesterday.

Delisio, cafeteria manager at the Agriculture Department in Southwest, normally buys $100 worth of half-pints for his 4,500 daily customers. Yesterday, he bought only $52 worth of milk for the light traffic. "We have about half as many customers as we normally do and cut back on inventory drastically, but that's to be expected in a holiday season," said Delisio.

At the U.S. Public Health Service in Rockville, which employs 4,000 people, the turnout was "heavier than normal" for a holiday week, said George Farah, a personnel officer. "Let's put it this way: There are a hell of a lot more cars in the parking lot than there were on Monday."

At the Bureau of Standards in Gaithersburg, spokesman Dick Franzen estimated that at least half the agency's 2,300 employes reported for work yesterday.

Bureacratic absenteeism took its toll on restaurants along Pennsylvania Avenue. "It's like a Monday," said Chris Chiam, the controller at the Bread Oven at 1201 Pennsylvania Ave. NW. "I guess a lot of people are still on the road. Nobody wanted to drive back yesterday."

However, tourists and Washingtonians-playing-tourists filled some restuarants in the Old Post Office and nearby. Moe Cheramie, his wife Stephanie, of Great Falls, Va., and their two children waited for a table at the Old Ebbitt Grill before a day of sightseeing at the White House and the Washington Monument.

"During the holidays we go crazy in my business. It's our busiest time of year," said Cheramie, who runs a seafood business in McLean. "The day after Christmas, we close down and I see the family."