The party was meant to be private, just a small informal birthday buffet, the 76 guests gathering at Woodlawn Plantation. But the finale of Nancy Reagan's birthday party last evening, returning to the White House to watch the fireworks at the Washington Monument, was to join her party with the most public birthday party of them all, the nation's.

By late last Wednesday, historic Woodlawn Plantation was still left guessing who was coming to dinner, as the party site had been changed from Dumbarton Oaks because of unidentified "conflicts," according to Mrs. Reagan's press secretary, Sheila Tate. And Ridgewell's, which was catering the party, hosted by longtime Reagan friends Betty Wilson and Marian Jorgesen, did not yet have the recipe for the guacamole. The recipe was Jorgensen's and Wilson's, adapted from Chasen's restaurant in Los Angeles, and submitted because Jorgensen figured Easterners have a lot to learn about guacamole.

Guacamole -- with tostadas, of course -- is a staple at Reagan Group parties, as is Chasen's chili, which in this case was flown in, frozen, from the restaurant. There was, however, plenty left for Ridgewell's to do: cold country-fried chicken, baby pork barbequed spareribs, corn muffins, herb bread, corn on the cob, thickly sliced beefsteak tomatoes with mozzarella and basil leaves, marinated cucumbers with onion rings, raw vegetable with watercress dip and homemade strawberry ice cream. Not to mention the cake.

The cake was as elaborate as a wedding cake: white pound cake with raspberry filling and white icing, four tiers high, its lacework in American-flag blue and its candles made of marzipan in American-flag red. All this topped with American flags and two pennants, one saying "Happy Birthday," the other saying "Nancy."

No doubt about the theme. Red-white-and-blue everything.The tablecloths were custom-made in dark blue to fit the eight 66-inch tables; over them, red and white checked cloths. Florist Ted Huffcut of Nosegay florist made eight centerpieces of red-white-and-blue hot air balloons, their baskets filled with American flags and pennants: Happy Birthday Nancy their message. Huffcut called the party a "totally coordinated look." Even the people were coordinated, the men wearing skimmers with bands in you-know-what colors, lettered with you-know-what. And for the women, gondoliers' hats: ditto.

While it had been reported that the Reagans have returned pomp and formality to the White House, this party was determinedly informal. The music was country and western, the serving pieces were baskets and wicker trays (except the silver bowls for the ice cream) and the white tent was filled with balloons -- tricolor, of course. "This is the way they all live in California," said Huffcut. But when Mrs. Reagan lived in California, her birthday parties were often in atypical places: on ranches, in an airplane, on horseback. This time such innovation was more difficult, what with her being the first lady and busy at the White House. Thus, instead of taking her somewhere for her birthday, this year, as Huffcut put it, her friends decided "to bring California to the Reagans for her birthday." BETTY WILSON AND MARIAN JORGENSEN'S GUACAMOLE (8 servings) 2 1/2 avocados, large ripe ones 1/2 cup grated onions 1 tomato, seeded and diced 1/2 pound bacon, cut in small pieces and cooked until very crisp Lemon juice, tabasco, salt and white pepper to taste

Mash avocados and combine with onions, tomatoes and bacon. Season to taste with lemon juice, tabasco, salt and white pepper. Serve with tostada chips.