One of the standbys of the holiday season is the office party, that foot in the door, that wedge in the log, that forerunner of a million other events where business puts on a pretty frock and tries to pretend it is social.

Another is the clutter of invitations propped on the mantel or stuck to the refrigerator door, which serves not only to remind the recipient that come Friday it is cocktails and then cocktails again and then dinner, but to tantalize friends who drop in and surreptitiously case the display, dreading to find they have been dealt Washington's cruelest blow -- social exclusion.

It matters -- oh, how it matters -- not because the uninvited guest wants to dance all night but because, like the official office party, so much of Washington entertaining has to do with business.

Business, whether diplomatic or financial, is business. Dressed in a party frock, it is business in drag. No one is fooled, but people keep trying to mate these different species, which says something either about the optimism or the density of the human brain.

There is a new book out called Company Manners by Lois Wyse (McGraw-Hill paperbacks, $6.95) which, in its attempt to provide guidelines for power eating, shows that it is time to give this hybrid the boot. For 1988, resolve that in your house there will be no more cross-overs. Business during the day, pleasure at night.

Here is Wyse giving advice on the business lunch: "Get a table that gives you maximum privacy when you're lunching with profit in mind. You should also have an idea of where your guest will sit and where you plan to sit.

"Things to avoid: a table facing a mirror, a table next to the rest rooms, and a table near the kitchen ...

"Always think in advance and plan your menu choice. You look silly and indecisive when you take five minutes to look at a menu trying to make up your mind about luncheon ... Never order food that is difficult to eat, requires your full-time attention, or squirts, slurps, and makes a mess ..."

Even worse are her comments on the business breakfast, that blight on what was once a cozy family affair: "It is always appropriate to invite out-of-town visitors to breakfast. It adds profitably to their working day, and since almost all of us are powered by the Puritan Ethic, we are comfortable with the thought that we begin the day with a business breakfast," writes Wyse, who then quotes a New York literary agent as saying, "It's hard to say no when somebody wants to get up early in the morning to see you." Is this true? Has the world gone mad?

There also are careful instructions about how to offer a drink but not to press it too strongly, how to be neither early nor late, at what exact point in the meal to bring up the fact that this meeting is to center on the deal not the veal. The reader begins to feel like Goldilocks, unable to reach the point where everything is just right.

Wyse also tells two stories that are very revealing about why business entertaining isn't. In the first, an employer invites a young worker to lunch. After ordering in a fast and decisive fashion, the worker tells the employer that the company needs five new computers. The employer, offended, replies that he had invited the younger worker to lunch in order to get away from the office, not to bring it with them.

In the second story, a woman who wants to sell her company has lunch with a potential buyer. Biding her time, she waits until dessert to mention her decision to sell. Alas, it is pie in the sky. Before she can pull out the books, he thanks her for this chance to get to know her better and leaves. Just as the first young man had assumed that when the boss asked him to lunch it was business, this one had assumed that the woman had included him in an unexpected but pleasant social event.

All of this confusion would stop if people would conduct business in offices. It is perhaps a Sisyphean chore to try to roll business back out of social life, but the very caution that is necessary to the conduct of business is antithetical to a pleasant and relaxed evening. In a social situation, one would never worry about taking too long over the menu; if a friend gets impatient, a friend will tell you to get on with it. Friends don't worry about having one drink too many and spilling an office secret or, just as bad when you're uncomfortable and on guard, spilling the sauce. How nice it would be if an invitation meant someone wanted to get to know you better.

It may not be possible to change the ways of the world, but you can change your own style. And if you must plan an evening where the business and social merge, make it a poker party. There at least your money and your cards are on the table.