When the lantern over the Capitol signifying that Congress is in session burns past 7:30 p.m. you can bet that in at least one grand house the hostess is worrying that:

Her guests will drink too much in an effort to assuage their hunger;

The cook will quit because the fish course is browning around the edges and the souffle' is collapsing;

War is being declared and even now the bombers are on their way.

From her Georgetown mansion, Pamela Harriman, one of the top two or three party givers in town, says, "It's the story of our lives. If I've invited senators and congressmen I expect the telephone to ring 15 minutes before dinner saying they'll be late an hour or two. At least they have a good excuse when a session goes into the evening.

"If I invite people for a private dinner, I ask them for 8 p.m. and we go in to dinner at 8:30. If the latecomer is the guest of honor, an important senator perhaps, I would wait 15 minutes more," she says. "Political receptions are the worst; sometimes the senator is very, very late. But people know the hazards."

Harriman, British-bred and once married to Sir Winston Churchill's son Randolph, says that in London people "are still very punctual. If you're invited 15 minutes ahead, then you know royalty is coming. The royal family is very much on time. I've never seen them late. And the same with prime ministers. Cabinet meetings seem to be over punctually. Of course, during World War II, there were emergencies, and a few times Sir Winston himself would have to be late to his own dinner. Still, the British are very punctual people. Americans are more casual. With my British punctuality, I spend most of my time waiting for others."

As she points out, the problem is not always with Congress. "I went to one dinner where an ambassador was to be the ranking guest. But he never came. The host apologized at dinner for his absence."

Last week, for a dinner party at the Embassy of Cyprus, Betty Wright and her husband House Speaker Jim Wright were late for dinner. "Half an hour -- oh, maybe as bad as 45 minutes," she admits. "The House was voting on the budget. Pamela Jacovides {the wife of Ambassador Andrew Jacovides} called when we weren't there, to say for me to come on if Jim was going to be later. I usually do, if he's going to be delayed more than 30 minutes, but this time his office said they'd adjourn by 8 p.m., so I waited for him. Mrs. Jacovides held dinner for us. She said she'd whipped up some tuna canape's, to help the guests wait, though she usually doesn't serve hors d'oeuvres."

Wright, who often entertains members of Congress in their new glass-walled banquet room in McLean, as the wife of the House speaker is doubly at the mercy of the closing gavel, as alternately hostess and guest. "You can't hold up everyone for one couple. I'll wait a half an hour."

The problem with Congress, says Wright, who knows it well, is "you can't predict when they're going to adjourn -- anything can and does happen on the floor of the House."

Countess Ulla Wachtmeister is a painter who spends more time these days creating ephemeral art -- her dining table centerpieces with sand, grass and growing flowers are famous. Now that Count Wilhelm Wachtmeister, the Swedish ambassador, is dean of the diplomatic corps, their entertaining is endless.

"If we have guests invited at 8 p.m., if the senator hasn't arrived by 9 p.m., I try to call the office, find out what they think, make an agreement. Perhaps there's no answer. I think, if I wait too long, the other guests are too tired and the kitchen is difficult. Still, Americans enjoy mingling before, because everyone goes home right after dinner. Each occasion is different. I think you have to be a little flexible. But a guest coming later will be embarrassed if he finds he's made everyone wait."

And there was the time the wait was due to an act of God and not an act of Congress. Sweden was giving a decoration to Sen. Claiborne Pell. A group of his constituents in Rhode Island hired a bus to come and observe this great event, scheduled for 6 p.m. -- only to be trapped in a snowstorm. The indefatigable countess "gave them a little nightcap" when they arrived at 10 p.m.

Both the countess and Linda Faulkner, White House social secretary, say members of Congress sometimes arrive on time, sit down to dinner and then their buzzers sound and they go back to an important vote. "It's so sad. They have a terrible workload," says the countess.

"During the congressional barbecue, most had to go back to vote. It's an occupational hazard. You learn to flow with it -- and serve buffet," said Faulkner.

Or announce that you follow the rule of the founding father. George Washington held dinner for only five minutes.