Washington's birthday may be celebrated on several different dates, Christmas can be one day or 12, and even on Halloween you have the choice of either tricking or treating. But this land, known more for its ethnic mix than its common culture, agrees on one thing one day of the year.

On Thanksgiving you eat turkey . . . unless you are a snob, an anarchist or a vegetarian. Or unless you are stranded on a mountaintop with an empty larder.

As I was.

For the first time, my family was spending Thanksgiving away from home, in a rented house on a tropical island. Palm trees seemed a fair trade for turkey leftovers, so we planned to arrive midday Thanksgiving and eat dinner in a nearby resort hotel. A four hour wait in the San Juan airport for the rain to abate did not dampen our spirits, and the light drizzle on St. Thomas served as no warning.

We did, however, realize that most stores on that island were closed for the holiday, so as soon as we saw one open we stopped to buy whatever supplies it had to sell: butter, apples, garlic, milk, wine and that bread named Vienna by somebody who obviously found glee in the fall of the Hapsburg empire.

By the time we arrived on St. John's island, the rain had picked up again and we were drenched with bad news. All the stores were closed, and it had been raining nonstop for weeks. We were lucky to have a house high in the hills, for the costal resorts were flooded. The caretaker drove us to our house in the jeep that was to be ours for the week, and on the way discovered that the roads were so eroded by the rain we dared not venture down again until the next day, after the caretaker had shored up the worst spots.

So there we were, in a house I could have drawn from my fantasies -- window walls and seven balconies overlooking what promised to be ocean and bay when the rain lifted -- with a single bag of groceries that included nothing even reminiscent of turkey. In fact, nothing was even reminiscent of dinner.

The kitchen was grandly equipped, with Sabatier knives and Danish cutting boards, but the oven was not working. So even if we had had a turkey, it would have had to be pan fried.

On the shelves we found a box of macaroni, a canister of grated Italian-style cheese, salt, pepper and cinnamon. What's more, we found candles. So everybody pitched in to make a feast. One son set the table, grandly, with place cards enough candles that every child could blow them out and light them again to his heart's content. Another son made garlic bread, crisping it in a toaster rather than in the oven. Our daughter, then seven, was in her glory concocting apple tarts from slices of bread rolled thin and doused with cinamon, sugar and butter, littered with apples, folded and fried in more butter. And while my husband arranged for the wine and our damp clothes to breathe, I set about making macaroni and cheese, the worst marcaroni and cheese I ever tasted, with plenty of butter and a soupcon of garlic and the entire container of that gritty packaged cheese dust that I had always scorned.

The children were the most delicious part of the dinner. They dubbed that dreadful pasta "turkey," and offered each other "another drumstick" and "a crisp bit of skin." The candles and the wine softened the edges of disappointment. And the apple tarts were truly good.

We have never had a happier dinner, and we had no problem, even without turkey, giving thanks.

Now, we realize we were merely avant garde, for if we are to believe only half of what we read, we know that under the Reagan administration macaroni and cheese is going to replace quiche as the food cliche of 1981. And, while Nixon's cottage cheese and ketchup never caught on, and Carter's grits remained a mere footnote on a few breakfast menus, macaroni and cheese is so versatile as to be appropriate for Saturday lunch or Thanksgiving dinner on a tropic isle. So you'd be advised to be ready for the holiday entertaining season with a recipe for authentic marcaroni and cheese. Not my island version. PATTY WEBB'S MACARONI AND CHEESE (4 to 6 servings) 1 1/4 cups uncooked elbow macaroni (or 2 2/3 cups cooked macaroni or other pasta) 2 tablespoons butter 2 tablespoons flour 1 teaspoon dry mustard 3 cups milk 3 cups grated sharp cheddar cheese 1/16 teaspoon cayenne (estimated) 1 teaspoon salt 3/4 teaspoon Worcestershire sauce Bread crumbs and butter for topping (optional)

Cook macaroni according to package directions, or in three quarts boiling salted water for about nine minutes, until al dente. Do this while making sauce. Elbow macaroni is traditional, but the larger size elbow macaroni is better.

Melt butter in heavy saucepan. Add flour and mustard and stir over medium heat for about three minutes to cook the roux. Remove from heat and gradually add milk, stirring with a whisk. Cook, stirring, until sauce thickens slightly. Cool a few minutes (so that the heat does not toughen the cheese and prevent it from blending into the sauce), then stir in the cheese, reserving a bit for the top. Stir over moderate heat until cheese melts, add remaining seasonings, and taste. The sauce on its own should be too intense for the cook's taste, since the macaroni is bland. If the pasta is undersalted, add salt to the sauce to compensate.

Combine sauce and marcaroni in a casserole. Top with the reserved cheese and sprinkle with fresh bread crumbs if desired, then dot with butter. Bake in a 350 degree oven until brown, or about 25 minutes.

Frills: a bit of shredded dried beef or ham in the mixture, or replacing part of the milk with cream. If the dish is to be frozen, add the topping after freezing and defrosting.

Testing Notes: Proportion of sauce to macaroni is higher than most recipes because we preferred a moister casserole.Don't be shy with the cayenne. It won't reveal itself.

It was my first year of college, far from home, to far to get there for Thanksgiving. After two months of dining hall mystery meats, the first holiday meal away from home was bound to be at least poignant so my roommate and I drove 40 miles to my aunt and uncle's house for the occasion. Two months' cultivation of freshman cynicism wafted away in the smells of home cooking as we entered. A real home, real children, a mother and a father -- anybody's mother and father. No hordes of acned strangers. Food being prepared in pots of civilized proportions, and smelling of known ingredients. The table, polished wood set with silver banded white plates, had flowers rather than the ketchup-bottle-and-sugar-packet centerpieces of our college feeding factory. There, on the sideboard were tray after tray of desserts: apple pie, pumpkin pie, nut-filled cookies, and my aunt's legendary babka, swirled with pale meringue, coconut oozing out the top. The smell of yeast and cinnamon did it. We meant to break off just a smidgeon, but it came off in a great hunk. We fell on it, stuffed ourselves with it. My uncle, never shying away from a prank, produced a pie server. We roared, giggled, doubled over, stuffed our mouths, immediately transformed from two sophisticated college students and a mature businessman to raucous children. Alarmed, my aunt ran in -- and collapsed in laughter. She ushered us to the table, gave thanks, and passed the dessert plates. Turkey could wait. IRMA CHARLES' BABKA (10 to 12 servings)

This lightly sweetened coffee cake is tall and handsome, appropriate for breakfast through evening. In slicing it, try not to shortchange anyone by leaving a piece with little or no filling. Dough: 1/4 cup sugar 1/2 pound butter 3 egg yolks (reserve the whites for filling) 1 teaspoon vanilla 2 yeast cakes 1/2 cup warm milk and 1/2 cup sour cream or 1 cup warm water 4 cups flour 3 egg whites 1 cup sugar

Cream 1/4 cup sugar and butter. Add yolks and vanilla and mix well. Dissolve yeast in warm milk, then stir in sour cream, or dissolve yeast in just warm water. Add flour and yeast mixture alternately to the butter-sugar mixture. Cover the bowl with a towel or waxed paper and refrigerate 6 to 10 hours. Roll out on a lightly floured surface to a rectangle about 16 x 20 inches. Spread with meringue made from three egg whites beaten stiff with 1 cup sugar. Leave a strip along one long edge clear of meringue, but otherwise spread meringue to the edges. Topping: 2 tablespoons cinnamon 1 to 1 1/2 cups raisins 1 cup chopped nuts (preferably pecans) 3/4 cup coconut (optional) 1/2 cup raspberry preserves (approximately)

Sprinkle cinnamon evenly over meringue, then raisins, nuts and coconut. Spread one thin line of preserves lengthwise down the middle of the dough, most easily accomplished by dabbing half-teaspoons of preserves next to one another.

Roll dough like a jelly roll, starting at the long end covered with meringue, and ending where the meringue ends. Finish the roll by picking up the unfilled edge and folding it over the top so the seam is on top. Cut roll into 6 equal parts. Grease a tube pan with a removable bottom. Place the six rolls cut side up in the pan, alternating larger and smaller rolls for balance. Finished cake will look prettier if the seams are inside, toward the center tube. Meringue should be oozing out of the slices. Cover pan with a towel and let rise 2 hours. Bake in 350 degree oven about 1 hour, until top is golden brown. Remove sides of pan and let cake cool, then remove rest of pan. Cut in thin slices. CAPTION: Illustrations 1 through 5, no caption; Pictures 1 through 4, no caption. $1