The population of Luxembourg, 385,000, was reported incorrectly in an article yesterday on a White House state dinner for the grand duke. In a guest list with the article, the name of French designer Mme. Celine was incorrect.

The White House State Dinner is not so much a party as an exact science of ritual. Details are set down to the minute, and planning begins three months ahead. Here, from the office of social secretary Gahl Hodges, are excerpts from an in-house schedule for last night's dinner for the grand duke and grand duchess of Luxembourg. Punctuation and capitalization are by the White House; the explanatory notes in parentheses are not.

7 p.m. Dinner guests arrive the East Gate.

7:10 p.m. Dinner guests are escorted by Social Aides to the East Room.

Guests are introduced in the East Foyer to the Members of the Press. (Note: Sounds pleasant, but what really happens is that guests have to walk by a mob of reporters who scream out embarrassing questions. Sometimes it dampens an evening. At a dinner for Republican governors, one reporter looked straight at the secretary of the treasury and asked him what state he was from.)

7:17 p.m. THE PRESIDENT and MRS. REAGAN greet first group of guests in the upstairs hall and proceed to the Yellow Oval Room for cocktails.

7:29 p.m. THE PRESIDENT and MRS. REAGAN depart the living quarters, on cue from (East Wing Staff Director James) Rosebush, via elevator and proceed to the North Portico.

(Note: This is important. Here, planning gets down to the second -- and, usually, works. Last night, after the grand duke and duchess left the Vista International Hotel, a staffer in the White House military office kept in contact on a two-way radio with a protocol officer in their motorcade. Rosebush gave the Reagans the signal to go downstairs as the limo pulled in the northwest gate; the White House knows by experience that the Reagans will get down there just as the limo arrives at the door.

(Except when they don't.

(A relative disaster once occurred when the radio failed, forcing the Reagans to go down early and stand around for three or four minutes with nothing to do in front of the press. The White House does not consider this dignified. So last night when the elaborate timing collapsed, the Reagans did the sensible thing. They went back inside.

(After staring at several arriving limousines, none of which held the grand duke and duchess, the Reagans smiled at the press corps. "Goodbye," said Nancy Reagan, waving as she and the president returned to the White House.

(Thirty seconds later they were back.

("Take two," said the president, smiling as the right limousines finally arrived.)

7:30 p.m. Their Royal Highnesses The Grand Duke and The Grand Duchess of Luxembourg arrive the North Portico via the Northwest Gate and are greeted by THE PRESIDENT and MRS. REAGAN. THE PRINCIPALS pose for photo on the steps and then proceed to the Yellow Oval Room via the elevator.

7:47 p.m. THE PRINCIPALS arrive on the State Floor via the Staircase, pause at the foot of the stairs for a press photo, and proceed directly to the East Room to Full Honors in the following positions (viewer's right to left):

THE PRESIDENT

His Royal Highness

MRS. REAGAN

Her Royal Highness

8:15 p.m. Dinner is served.

9:20 p.m. Dessert is served, accompanied by the Strolling Strings.

10:22 p.m. Dance performance begins.

10:55 p.m. Performance concludes.

Dancers take a bow.

Miss Twyla Tharp joins the troupe for a bow.

11:05 p.m. THE PRESIDENT and MRS. REAGAN return to the Grand Foyer. Dancing in the Grand Foyer.

(Note: After a dance or two, the party's over. No lingering at the White House. "By 11:30," says Hodges with relief, "we're usually shut down.")