While the rest of us drift along under the dogwood trees and grow misty at a patch of daffodils, caterers all over Washington are getting on their marks.

The sentimental season, which makes us languid, makes their lives hectic, jammed full of June brides and graduations, anniversaries and proms. April through June is one of the year's two major party seasons: the time when people who have never before used a caterer--and will never again--pick up the phone and ask for help. Washington is considered one of the best cities in the country in its choice of caterers, and some are called on to do special parties in other cities.

Knowing what to ask can make a difference in what you get, and most important is to know what you want. The date can be particularly important.

"Washington is notorious for everyone scheduling a party on the same night," says Bill Marsh of Braun's Fine Caterers.

Most caterers prefer a couple months' notice. But, says Bruce Ellis of Ridgewells, "We've had people wanting us to cater a reception for 200 to 300 people on two days' notice. We do it, but we don't like to. That usually happens when someone comes into town unexpectedly and the host decides he has to be entertained."

Once you've chosen a caterer, the rest is between you and your pocketbook. Most caterers will do everything from a minimum of providing food to a maximum of arranging for the china, waiters, liquor, florists, musicians, limousines, tents, photographers, etc. The more you have the caterer do, the more it will cost, of course. But it may be worth the extra money to know that when the bartender doesn't show up, or a case of glasses arrives broken, the problem isn't yours, it's the caterers'.

Generators, ovens, refrigerator trucks are all things a caterer can bring into play to avert disaster. Caterers will tell you how many people you should expect. In certain months, they have discovered, a higher percentage of people accept invitations.

"In June, especially the first three weeks, 80 to 90 percent of the people invited will attend," says Carl Longley of B & B Caterers. "People seem to reserve certain times of the year to go to parties. It's the same thing the weeks before Christmas."

Caterers also know what kinds of liquor your guests are likely to prefer. "North of Baltimore, people drink more rye and scotch, south of the bridge, it's bourbon," claims John Orcino of Avignone Frere s.

Most important, they will help you choose the food and warn you when your favorite menu is likely to be a disaster.

The word usually is no soup, unless guests are going to be seated, or it's to be drunk out of mugs. And for buffets, avoid dishes that require a knife: One utensil is all people can handle when balancing a plate.

"Sometimes we have to dissuade people from things that wouldn't work," says Ellis. "If they are very insistent and it's really not a good idea, we may have to say, 'We wouldn't be proud of serving it.' It can get testy, but if we can do something we will.

"One customer was giving a party in a tent in the Virginia countryside in the middle of summer. She wanted baked Alaska for dessert. We have trucks to keep things frozen, so we managed it. If we think we can keep it from being a disaster, we'll do it."

A caterer will visit your home and tell you whether you need to rearrange the furniture or rent tables and chairs. They will also work out the traffic flow and place the bar or bars accordingly. Caterers recommend placing the bar where it can be seen as people arrive for a large party, with the second bar--if there is one--in sight of the first, so that people will move on if one becomes overcrowded.

Marsh of Braun's Caterers recalls the time they catered a cocktail party with a guest list of 20,000. "People had to be let in at seven-minute intervals. We ordered cheese by the ton, 300,000 hors d'oeuvres and the vegetables had to be cut up into 55-gallon trash cans--new ones, of course."

The most difficult parties to cater, says Orcino, are the ones "where the customers play too close to the line, saying that they're having 10 people for dinner and at the last minute calling and saying it'll be 12 and could we please deliver two more plates. It's also difficult when people try to mix and match their equipment with ours."

"Customers put emphasis on different things," says Ellis. "One may want to skip brand-name liquors and put more money into flowers and food. Another may want the brand names and be less extravagant with the food. We talk to the customer to eliminate what they don't like and work out what they do."

Your caterer also can help you make your party less conventional. "We want people to be surprised," says Marsh.

Caterers are in competition and try to come up with different ideas. Even so, they find that like every dog, every dish has its day. Right now it's veal, favorite of President Reagan. And white wine is still the partygoers' favorite drink.

If, after checking with a caterer, you find you can't afford the service, you might get a copy of The Complete Book of Entertaining, by Elizabeth Post (Harper & Row). The first section of the book contains information that will help you figure out how many people will comfortably fit in a given space, and how much they'll eat and drink.