Never, never should a pseudo-breakfast be called (shudder) "brunch," Vogue's Book of Etiquette decreed in 1948. The author and then-associate editor, Millicent Fenwick, went on to scorn "that modern invention with the horrid name."

In the intervening years Mrs. Fenwick has moved from the world fashion to the political arena here in Washington, and has done it with her typical great style and familiar trademark - a small, properly elegant pipe. In the same interval, that weekend midday meal has become so popular that it no longer appears within quotation marks.

And what used to be defined as a "morning meal midway between breakfast and lunch" has now slipped further and further down the clock to something between lunch and the cocktail hour. Restaurants may start brunch service at 11, but you can bet your Bloody Mary that the chic crowd won't arrive before 1.

With brunch at home intended to be a rather informal meal, the host or hostess needn't be as fussy about transfering jam into pretty little dishes, curling butter, and all the other niceties that have gone the way of hats on Sunday and white gloves to tea.

But to insure a smooth schedule and calm nerves, there is still one rule that should not be ignored: Don't experiment on guests. One hostess who didn't heed that maxim had a problem on her hands when thirty-some guests arrived for a 12:30 brunch. Bloody Marys flowed and flowed and flowed, and by 3 o'clock the crowd was restive and hungry. The harried cook dashed into the living room, clutched a knowledgeable guest by the hand and took her back to the kitchen. Near hysteria, she pointed to the oven. "What's wrong with my souffle? It won't rise and it's been in for an hour." As the expert looked at the bathtub-size dish slowly coddling the eggs and cheese, she realized that this was not the moment for a basic lesson in cooking. Her advice was: "Call it a cheese mousse and serve."