Five years ago it was probably the worst kept secret in the country. Tom Brokaw almost blew it when he blabbed over the "Today" show that 100 people were going to help President Reagan celebrate his 70th birthday that night.

"My," Nancy Reagan told her husband, doing her best to cover up, "that Tom Brokaw certainly exaggerates."

She won't have to worry about blabbermouths ruining her surprise this year because President Reagan already knows about his 75th birthday party. The same four California couples who gave the 1981 party -- the Walter Annenbergs, the Armand Deutsches, the Earle Jorgensens and the William Wilsons -- also will give this one.

Beyond that, pickings are going to be slim on who's coming to dinner, what's going to happen there and who's giving what. According to Elaine Crispen, Mrs. Reagan's press secretary, this party, the day after the president's birthday, will be "strictly private" and not open to press coverage.

That means no guest list issued by the White House. But if the one for Reagan's 70th birthday is any guide, some faces likely to be in the crowd are the William Buckleys, the Henry Salvatoris, the Francis Albert Sinatras, the Holmes Tuttles, the Jack Wrathers, Betsy Bloomingdale, Jane Dart, the Jacqueline Humes, the James Stewarts, Stuart Spencer, the William French Smiths, the Michael Deavers, the William J. Caseys, Ted Graber, Jerry Zipkin and Julius Bengtsson.

The only partygoers confirmed by the White House are some Reagan family members. They will be Maureen Reagan and her husband, Dennis Revell, and Dr. Richard Davis and his wife, Patricia. Unable to make the party, either because of illness or conflicting schedules, are the president's brother, Neil Reagan, and his wife Bess, and the Reagans' other children, Ron, Patti and Michael, and their spouses.

Another party planned for Friday is the annual celebration of First Hairdresser Julius Bengtsson's "19th" birthday. And who else would be tossing it but Second Hairdresser Robin Weir?

Announced a spokeswoman for Weir yesterday, "Julius will be here on schedule." Which goes to prove how really momentary that flare of temper was that sent Bengtsson stomping out of Weir's P Street NW salon recently.

The question now is will he or won't he bring back all the personalized celebrity photos he took from the VIP Room walls? Alas, only VIPs will know.

The First Couple won't be losing any sleep over Ronald Prescott Reagan's appearance this week on "Saturday Night Live." If Ron does have a couple of things to contribute to the show's traditional lampooning of his parents, they won't know about it until Sunday. That's when they plan to watch a videotape of the show, according to Crispen.

Ron, incidentally, has been named a contributing editor of Playboy. Says Executive Editor Barry Golson, "It means that we take the association seriously and also that we have first refusal on everything he writes."

Golson says young Reagan won't get rich on what Playboy pays -- "basic free-lance rates." But he says Ron is a talented writer "who can write stuff that makes you laugh out loud. He probably had to prove himself more with us, considering our politics, but he's made converts out of us."

Ron's article on the Geneva summit is scheduled for the April issue. It won't be your run-of-the-mill summit stuff. Golson says it's funny.

American moviegoers know her best as Catherine Deneuve, the glamorous French actress. The French also know her as the current model for Marianne, the woman's figure that is the personification of the French Republic. In effect, Deneuve will be playing both parts while she's in town this week. Tomorrow night she'll be the glamorous actress, starring at a gala benefit arranged by The Friends of Vieilles Maisons Francaises at La Maison Francaise to open the French Film Festival, Friday through Feb. 20 at the K-B Janus. Her own film, "L'Africain," will be shown at the gala.

On Thursday, President Reagan's 75th birthday, Deneuve will be the symbolic representative of French President Francois Mitterrand when she joins French Ambassador Emmanuel de Margerie as he presents a crystal model of the Statue of Liberty to Reagan in the Oval Office. With them will be former Ambassador Francois de Laboulaye, now president of the official Franco-American committee for the celebration of the centennial of the Statue of Liberty.

Created by Saint Gobain -- France's oldest glass-making firm, which dates back to Louis XIV -- the 22-pound statue is one of 55 being sent to American groups and officials, including Vice President George Bush, House Speaker Thomas P. (Tip) O'Neill and the 50 state governors. Tennessee's Gov. Lamar Alexander will accept the governors' statues when they all come to town later this month for another of their conferences.