In year-end Washington, a quiet solemnity

The White House, on the inside, is richly decorated for the holidays. Banks of poinsettias warm the hall inside the East Entrance; wreaths (with teddy bears inside) line the State Dining Room. Antique dolls and cut greens are everywhere. The 20-foot tree, hung with small toys and hundreds of handmade ornaments, appears to have been shipped in from the Victorian era.

But the outside of the President's residence is solemn as a dark winter coat. There are few special lights, no ribbons, no wreaths. Instead, workmen have been installing banks of earth and shubbery, surrounded by concrete slabs, that block gates to the grounds.

This juxtaposition seems to match the city's mood. While festive in private, the official Washington, D.C., is not outwardly exuberant this holiday season.

There is, for one thing, the matter of Lebanon. Like any company town, Washington usually has one issue that is the primary topic of conversation, and right now the question most discussed at parties and lunches seems to be ''should we bring the marines home?''

Twelve of the 22 serious questions at President Reagan's Dec. 20 press conference dealt with the Middle East. Congressional administrative assistants, who as part of their job must be political early-warning systems for their bosses, tend to agree that Lebanon will dominate all other issues when Congress reconvenes.

The antiterrorist actions now being taken around town, in the aftermath of the bombing of US facilities in the Mideast, have also had a sobering effect. The Park Service crews installing barriers at the White House have been conspicuous, laboring past the evening rush hour with the aid of floodlights. (Among the less-well publicized precautions are the closing of pedestrian tunnels under the Pentagon and the fact that, after Jan. 1, autos will no longer be able to stand in front of the White House.)

And Congress has not been around to liven things up - at least, not officially. Legislators fled for home on Nov. 18, the earliest nonelection-year adjournment in almost 20 years.

Since then, however, many have sneaked back into town for a little quiet time in the office, unbothered by constituents or lobbyists.

''He's not supposed to be here,'' groans a congressional aide whose boss's reappearance was preventing her from finishing holiday preparations. ''He's supposed to be in New Jersey.''

So, all-in-all, official Washington has been a quiet place the last few weeks. Those parts of the city which revolve around the federal government are not filled with outwards signs of the seasons.

There were lots of office parties. Residential neighborhoods are festooned with lights and bustle with shoppers. But a walk through the core of the city - from the White House, through the downtown shopping area, and back up the Mall - reveals nary a single outdoor decoration.

There is hardly a sprig of pine on the Treasury, the Commerce Department, the Justice Department, or the FBI. There are no garlands or giant candy canes hung along F Street, department store row. There weren't even any Salvation Army Santas this December.

The one exception is the National Christmas Tree, on the Ellipse. Around 40 feet tall, it changes color from white, to pink, and back again, as regularly as a stoplight. It is surrounded by a host of small trees with pink lights, symbolizing the 50 states and all US protectorates. A burning Yule log and a stable of reindeer from the National Zoo complete the exhibit.

''They look like cows,'' says a small girl, staring intently at the reindeer.

''Well dear,'' replies her mother patiently, ''why don't you come look at their front end, instead of their behinds?''

A small boy eyes a reindeer with one antler. ''I can't see Rudolph,'' he says , obviously disappointed with this poor specimen. ''Where's Rudolph?''

Central D.C., however, may soon have at least one traditional, communal holiday celebration. City officials announced last week that they will throw a free New Year's Eve bash at the Old Post Office on Pennsylvania Avenue, a party they hope will become an annual event comparable to New York's festivities in Times Square.

In New York, point out D.C. officials, people just stand around in the cold and wait for the new year to arrive. In Washington, they'll be indoors and entertained.

There will be soap opera stars, a country and western singer, an orchestra for dancing, and the disco group Weather Girls. The arrival of the new year will be marked by lowering a giant, three-dimensional stamp down the Post Office clock tower.

''The downtown area,'' said deputy mayor Curtis McClinton at a press conference announcing the move, ''is alive and kicking.''

One Washington wag, however, suggests the city might attract a bigger crowd if it hailed 1984 by lowering a giant, three-dimensional PAC contribution down the Capitol dome.