In Washington, the seat of government, the nation's capital, the town which is normally busy running the country, this is the week that isn't.

Congress is in recess. The President is in Poland. The city's judges are on vacation and large numbers of Washington's 350,000 bureaucrats are on annual leave. From the Capitol to the White House to government offices scattered hither and yon, quiet has descended on official Washington this week between Christmas and New Year's Day.

Waiting for an elevator in the nearly deserted headquarters of the Department of Health, Education and Welfare, secretarial worker Ida Bryant had a succinct explanation for it all. "Around here," she said, "Christmas begins after Christmas."

Up almost to Christmas Eve, the government grinds along at its normal pace, with most people staying on their jobs.

But between Christmas and New Year's Day, everybody in government looks at the calendar and discovers - sometimes with a sense of panic - that just a few days remain to take accumulated leave. For there is a saying about leave time in government: use it or lose it.

Tours of several government buildings and telephone calls yesterday to various agencies revealed that thousands of civil servants appear to be busily engaged in using up their time. Rows of desks are vacant and normally clattering typewriters are silent.

One reporter, making a routine inquiry to the Interior Department, was given the name of five potential sources of information.Four were on leave, and the fifth out to lunch.

The Civil Service Commission, which oversees the government's personnel administration, said there were no reliable statistics on the number of authorized absentees, but a spokesman said the figure would be high.

Martin K. Schaller, executive secretary of the District of Columbia government, which follows civil service personnel rules, estimated that 40 per cent of the city's work force was off duty.

Their absence was reflected by a holiday atmosphere throughout downtown.

Along Pennsylvania Avenue, where federal workers usually stride briskly during their noon breaks, they were walking leisurely. Traffic seemed lighter than usual. The Metro "lunch bunch" that usually crowds the subway trains at noontime had thinned out. Patrons dawdled over meals in restaurants with tables uncommonly empty.

At HEW headquarters, a hand-scrawled sign, accurate in content if flawed in syntax, was taped to an assembly room door: "The meeting has been cancel."

Upstairs, in HEW's management services office, 4-year-old Deneshea Beynum sat at a desk playing with a rubber stamp and an ink pad. "I brought her because the work is kind of slow," her mother, Geerie Beynum, explained.

At the Voice of America, whose broadcast facilities occupy part of the HEW building, it was business as usual.

Pedro Kattah, typing a newcast in Portuguese for listeners in Brazil, explained: "The program has to go on every day. No break."

On Capitol Hill, with Congress adjourned, there were no signs of legislative activity. In the Senate press gallery, only two press releases were displayed in a rack that frequently holds dozens.

"If you're looking for a place where nothing is happening, you've come to the right place," quipped John Fogarty, correspondent for the San Francisco Chronicle.

John C. Larnett, a press and administrative aide to Sen. Gaylord Nelson (D-Wis.), occupied himself by reading a backlog of documents stacked on his desk. "This is the slowest week of the year," he said, noting that only half the staff was on duty.

With President Carter flying off yesterday morning on a nine-day foreign journey, the White House press office was equally somnolent.

"Only a few routine calls, and a reporter now and again floats in - that's about all," reported Mark Henderson, an associate White House press secretary.

Unlike Congress or the White House, where policies on employee leave tend to be informal, the rules in the bureaucracy are rigid. They are based on something called the "leave year." And the 1977 "leave year" is unusual - it will expire on Dec. 31, the same day the calendar year ends.

The Civil Service Commission spokesman explained the situation this way: The "leave year" is based on the government's two-week pay periods, which always begin on a Sunday and end on a Saturday, and don't often come out even with the dates on the calendar. Sometimes, in fact, the "leave year" ends as late as Jan. 12 or 13 of the following year.

But this year, the coincidental ending of the calendar year and the "leave year" on Dec. 31 has increased the number of workers who must use their time off in the period between the two holidays.

One government worker who hasn't noticed any less activity on her job is an elevator operator at the Distric Building, Washington's city hall.

"Maybe there aren't as many of them," she said, speaking of government employees, "but I'm telling you, they're still keeping me running up and down."