Courage! Only 15 days until the end of May!

Don't bother to count your fingers while reciting "Thirty days hath September... . " Old folk rhymes are all very well, but this year in Washington, May does not end on Thursday the 31st.

Thanks to at least two important foreign visitors, May this year extends through June 11. Mikhail Gorbachev's summit with President Bush will keep all the diplomatic types hard at work until June 3. And then, after everyone's had a night's sleep, Mexican President Carlos Salinas de Gortari will come to Washington on an official visit from June 9 to 11. While he's here, he and Mexican Ambassador Gustavo Petricioli will inaugurate, with suitable festivities, the new Mexican Cultural Institute in Mexico's former chancery on Meridian Hill.

So obviously no one can leave town until at least the middle of what used to be June before May engulfed it.

It wasn't always this frantic. In the Good Old Days, everyone could head for the hill stations when Lent officially ended the social season and Congress went home for the summer. This rule virtually eliminated May from Washington's calendar.

No more -- this May has had enough parties to supply not only June, but any ordinary year. Chronicle's annual official survey of weary feet, pounds gained and dry cleaning bills, shows indisputably this month has already had more days in it than usual -- otherwise how could we have fit in all those parties?

Why do they do it? What excuse is there for giving all those parties in May -- a month notorious for overbooking of caterers, beauty salons, husbands, etc. Not the weather -- certainly. Not a day has passed, it seems, without thunderstorms or threats of thunderstorms.

At the Folger Shakespeare Library's gorgeous "La Fete des Ballons," held on the Belgian Embassy's lawn, Ann Greer, the public affairs coordinator,explained, "It's because everyone leaves Washington at the end of May."

Ambassador Herman Dehennin, who said he liked the Folger guest list, was happy to show off the embassy's lawn at its best moment. The party attracted a civilized 250 guests, many in dresses from the attic. The Count and Countess Chandon, whose Moet et Chandon vintners have seemingly donated all the champagne for the month's parties, came to rejoice in the consumption. Maynard & David Caterers dispensed Belgian waffles (what else?), and ice coffee with cream and Belgian chocolate (what else?). Mimi Dehennin gave everyone a sweet farewell with chocolates wrapped up in a Belgian embroidered handkerchief.

The National Gallery of Art alone this month has had four major dinner parties, each of several hundred people, triumphs of artistry over numbers to celebrate shows of major collections. The Gallery's excuse? So the summer visitors will have an art feast.

This May has also seen the revival of the ancient custom of the private party, in which (amazingly enough) friends are invited to see friends.

Art consultant Lee McGrath's excuse for her party for 100, at the McGraths' multi-level house overlooking Rock Creek Park, was simple: "Dorn {her husband, a professor at George Washington University} was born on May 16." The surprise party -- how did she keep it from him? -- was complete with a band, three original cabaret songs written and performed by friends, and dozens of centerpieces, created by Mrs. McGrath, each one celebrating some great event in his life (blue doll shoes on a map of Peru, for instance, remembering a trek), and the huge cake was surrounded with effigies relating to his work as a community designer and planner.

Lillie Lou Rietzke -- whose garden dress copied from Renoir's painting of the girl in the swing attracted comment at the British party for Prince Philip last Saturday -- gave a party herself this past Monday. Her excuse: "Maryon Allen {the widow of the late Senator James Allen} is visiting."

Frankie Welch's fancy lunch Wednesday at her historic Alexandria house honored Doreen Whitney, chairman of the Friends of the First Ladies. The group raises money to conserve and display the gowns of presidential consorts at the National Museum of American History. Louise and Arnold Sagalyn's reason for inviting a number of people who may or may not have been concealing state secrets Tuesday was to acknowledge publication of Rod MacLeish's new thriller "Crossing at Ivalo." Concern Inc., the environmental activists, lit candles (biodegradable?) for its 20th anniversary. The founders -- Cynthia Helms, Nancy Ignatius, Joan Shorey, Peggy Mickey and Aileen Train -- could hardly believe two decades have gone by.

Historian Joan Challinor and environmentalist David Challinor with their daughter Mary Challinor and her husband Henry Richardson gave an honest-to-goodness dance at the Potomac Boat Club "on the last day of the year when it was cool enough to dance in Washington," Joan Challinor said.

As to what will we do when May's over, the answer came at a party given by Glaucia Baena-Soares, co-chairman of the Sixth Annual Hispanic Designers Fashion Show & Benefit. Fernando Sanchez, who will receive the Moda award at the end-of-summer event, said his clothes are designed "horizontally -- meant for resting on the chaise longue."