It is Friday night, the beginning of the weekend after Labor Day.

In the City and its close-in suburbs, 70,000 or so adults are attending parties, most of them informal gatherings of friends. A Small portion, perhaps 7,000 to 10,000 are at the kind of social gatherings Washington is best known for, cocktail parites.

In approximately 200,000 households, people that left or are getting ready to leave town for a night or a day or the entire weekend. Most of these are families many heading for an end-of-seson trip to the beah despite the light evening rain. There are also perhaps 20,000 unmarried couples, most of them under 30, going off together.

And some 100,000 people the weekend will be hardly any weekend at all: they will be working at all-time jobs the way they always do, and another 100,000 will work at least one weekend day. About 6,000 of these weekend workers are plying second jobs to supplement their incomes.

These figures come from a Washington post telephone survey of 1,012 people in the Washington metropilitan area conducted two weeks ago. In what were generally 10-to 15-minute conversations, Post interviewers asked questionaimed at determing what the nation's capital does on weekends of general and what it did the weekend of Sept. 10 and 11 in particular.

here are too many pitfalls in such a sruvey to assent that each set of figures is exact, or even correct within a few percentage points. But there is little doubt that the survey technique provides the best possible unsight into the community's activities, short of interviewing every one of the 1.8 million or so, people 18 or older who live in the places polled.

Perhaps th emain finding is that, with al the diversity found in this area and despite sharp contrasts in educational and economic circumstance, the simalarity in people's activities far overwhelms the differences. That is true for the young and the old, black and white, city-dweller and suburbanite, those whose lives center around their families and those who live far from their families or in broken homes.

Even comparisons between the most aftluent and the poorest, while marked by sharper differences than found elsewhere, show almost exactly the same proportions of people going shopping, attending church, watching TV, going to movies, visiting with friends, sightseeing downtown or grabbing a bite in a fast-food restaurant.

For all adults in the Washington area one weekend activity overwhelms the others watching TV, Four of evey five people surveyed watched at least a few minutes of TV over the weekend of Sept. 10, and half of those who watched - something like 700,000 people, that is - watched for more than four hours.

There was no Redskin game televised that weekend, but the TV adaptation of John Ehrlichman's novel about the Nixon years was in its concluding nights. The average viewer spent 5 1/2 hours before the set, according to the survey, and more than one person five watched TV at least nine hours.

Watching TV the longest were the youngest people surveyed and the oldest, especially the unemployed, the widowed and the retired.

The three other great shared activities of Washingtonians - as with Americans everywhere, no doubt - seem to be shopping, visiting with friends and puttering around the house.

More than half of those surveyed said they went grocery-shopping that particular weekend, and more than four in 10 shopped for items other than groceries. Working women were much more likely to go shopping than housewives as might be expected.

Six of evy 10 questioned visited with friends with some sharp differences noted by age and especially by marital status. Three in four single people, and three in four of all people under 30, spent sometime with friends. Only half of those who are married and living with their spouse and children spent time with friends

One U.S. senator whose phone number popped up at random along with the other said he generally goes home to campaign whenever he can on weekend;s a 25-year-old single woman, a newcomer who lives with roommates in Cleveland Park, said she would fly to California every weekend in the summer if she could.

But by a plurality of 43 to 37 per cent, those surveyed said they generally prefers staying home to going away on weekends, and the weekend after Labor Day was a perfect one for staying home.

Friday evening's slight rain turned to warm and pleasant days Saturday and Sunday. At One of every five single-family houses lawns were mowed; at one in 10, people surveyed their end-of-season vegetable-garden havest at one in four, other yard work was being done.

In half of the area's approximately 950,000 households people spent time at indoor cleaninf or other housework; some 30,000 people washed their cars.

One Washingtonian in four was mare active spending an average of 3 1/2 hours engaging in sports. The survey figures suggest that as many as 100,000 people found their way onto tennis courts during the weekend, and that half that number took part in the great 1970s jogging craze. Perhaps 250,000 tossed a frisbee or played football, basket-ball, baseball or softball.

And that number was a small compared to the 150,000 or so who spent an average of more than four hours at hobbies - from banging congo drums to tinterior decorating to working thousand-piece puzzles.

Among the most common hobbies for women were knitting, sewing, macrame, crcheting and similar activities. Almost 15 per cent of the women surveyed - which translates to 130,000 women in the community at large - repotted doing some such handicraft.

As for men, the survey suggests some 22,000 workde on cars, alike number in carpentry or carpentry-related hobbies, and another 22,000 spent at least part of their weekend playing a musical instrument (as sis about the same nimber of women)

Just over a third of those sruveyed said they attended religious services during the weekend. There sere sharp differences in patterns of attendance, however, with blacks much more likely to go to church than whites, younger people much less likely than older people, and men less likely to attend than women.

Perhaps one of the more striking fundings of the survey was the extent of book-reading reported. Leaving students aside, and omitting those who migh have counted Bible-reading in church, 51 per cent of those surveyed said they spent at least sometime during the weekend reading a book. Nine in 10 read for pleasure. One-third said they read fiction, 49 per cent ninfiction, and 18 per cent said they read both.

One in 10 people surveyed reported going to a movie the weekend of Sept, 10, with more than half of them falling in the 18-to-30 age group. Six per cent said they went to a live theatrical or musical evetn, with more going to hear rock music thatn any other single type of live entertainment.

Another tidbit turned up is that hlf a million people ate at fast-food restaurants over the weekend, about 30 per cent of them at a McDonald's.

Married people with children generally prefer staying home to going away by a 48-to-31 margin.

Staying home, of course, sometimes is simply inviting trouble. On the weekend of Sept. 10, some 10,000 Washingtons found themselves acting as sightseeing guides for out-of-twn friends or relatives.