If your mailbox is not yet filled to overflowing with invitations, you might as well leave town or reconcile yourself to spending the fall with a bad book.

For this week is the new year, the beginning of the fall entertaining season, the time to strike up the band. Forget what everybody has told you about Jan. 1. In Washington, the real year -- just as the school year -- begins following Labor Day.

The House comes back on Sept. 5. The Senate on Sept. 10. All Washington observers know what that means -- back to business as usual. Everybody knows the theaters, art galleries and museums will open new shows this month. So that takes care of Sundays. The Birthdays But only the chosen are invited to an assortment of other September parties. (To foil the unworthy, or at least the uninvited, dates will not be listed here for invitation-only parties.) Both the Smithsonian Resident Associate Program and the Washingtonian magazine will light 25 candles on their birthday cakes at separate, but equal celebrations. (Notice how no one ever celebrates anniversaries in August, but then nothing ever begins in August. Those who, through no fault of their own, were born in That Month -- namely one of the young Chronics -- should lie and choose a birth date past Labor Day, when everybody's back from the beach. As precedence, consider that for years members of the British royal family have chosen birthdays when strawberries are in fruit, suitable for garden parties.) Despite the recession, rumors of a depression and the closings of Garfinckel's and Arthur Adler, opportunities for conspicuous consumption continue to appear, opening after Labor Day and before Christmas, naturally.

The Italian Ambassador and Mrs. Petrignani are feting Alda Fendi, who's coming to town to inaugurate the Fendi Boutique. Fendi's opens to the public Oct. 1 at Fairfax Square. And Hermes is giving its own invitational petit de'jeuner for those most likely to be left holding the bag (after a suitable stamping of their charge plate, of course). The Friends of the First Ladies, the organization headed by Doreen Whitney to raise money to restore and reinstall the Smithsonian's collection of First Ladies' gowns, will benefit from a cocktail party at Hermes on Sunday, Nov. 18, with a gala following on the third floor of the Fairfax Square building. Hermes's public opening at Fairfax Square is Nov. 19.

The Friends are also having a September party -- at the Museum of American History -- for their friends and those of Carl Sferrazza Anthony, whose new book, "First Ladies: The Saga of the Presidents' Wives and Their Power, 1789-1961," is being published by William Morrow. The caterer, Eric Michael, is cooking up recipes from First Ladies for the party.

The wife of the president of Egypt, Susan Mubarak, will help Egyptian Ambassador El Sayed Abdel Raouf and his wife, Farida El Reedy, and art collectors Daniel J. and Judith Terra inaugurate the Foundation to Preserve the Antiquities and Monuments of Egypt at an invitational tea in September.

So be warned -- you must cease and desist writing your novels on company time, bringing your children to the office, going without ties, leaving off your pantyhose and other manifestations of vacation time or vacant minds. Run through your checklist: Pick up your dinner jacket or evening dress from the dry cleaner. (Sen. Robert Byrd, who has earned the right to eschew such fripperies, is exempted.) Find your diary and quit using it for your grocery list, or wishful thinking. Locate your envelope opener. Start checking your messages. (How are you and Miss Answer Call coming along?) Rearrange your thoughts in a suitable fall mode.

The New Year Is Here!

For a more complete list of September events, see Robin Groom's Social Calendar on F2.