Tomorrow morning all over Washington, the populace will be transformed into April Fools. Across the ocean, in Paris, people will awaken to find themselves un poisson d'avril; in Scotland the moors will be thick with gowks and in India there will be bloom Huli fools.

And a very good time will be had by all, as people inform each other of false promotions, invite friends to non-existent parties and tell fabulous stories whose punch line is April Fool!

In France, un poisson d'avril is celebrated, appropriately enough, by people attempting to pin small, gold-foil fish on their victims' backs. And if you wander down to Montmarte or one of the student quarters, you will be surrounded by people getting joyfully drunk, singing loud songs and kissing strangers. Why not, when one has been publicly labeled a silly fish?

There are almost as many theories on the origins of April Fool's Day as there are April Fools. Some antiquarians trace it to the Roman festival Hilaria, a celebration to Cybele held at the vernal equinox. Since festivals lasted eight days, the celebration culminated in a great burst of gaiety around April first.

Another theory holds it a relic of the Roman Cerealia, when Prosperpina, sporting in the Elysian meadows, was carried off to the underworld by Pluto. Her mother Ceres, hearing Prosperpina's screams, went in search of her daughter. But she, like the rest of us, had gone on a fool's errand, looking for the echo of a scream.

Arranging fool's errands has always been part of the day -- sending a child to the store for a quart of pigeon's milk or half a pound of inkle -- so I suppose you could call up your friends and invite them to an April Fool's party and then, when they come, not be there. But then you wouldn't have any friends to celebrate the rest of the year with.

Instead, fool them with food. Though few of us could manage 4 and 20 blackbirds baked in a pie, the idea of a surprise under a pastry crust serves well. Bake a meat pie in an oval dish, but decorate the top crust to look like a fish. Or cut out a gold-foil fish and tuck it around a platter of hamburgers.

You could make a raised pastry coffin -- the kind that usually holds a pork pie -- and fill it up with candy. A dinner to fool the eye.

For dessert there already exists a dish appropriate to the holiday -- a Fool. The Cookery of England by Elisabeth Ayrton gives a simple recipe: Blend 1 pint well-sweetened, sieved fruit puree, 1 pint of double (or whipped) cream, and extra sugar to taste. Put the mixture in a bowl or individual dishes and chill. You can blend it altogether so it's one pastel shade, or blend it lightly so it's striated with color.

Serve the dessert with ladyfingers or pieces of sponge cake, or put the pieces of cake in the bottom of the bowl, pour the fruit mixture over it and top with whipped cream. In which case, it is less of a fool, and moving toward being a trifle.