"Mommy, I want to change my birthday," Katy announced. "I'll have it now instead of next March." You get only one birthday and that's on the day you're actually born, I explained. But four-year-olds are persistent.

"Okay," I heard myself saying, "we'll have an unbirthday party."

"What's that?" Katy asked, nonplussed by winning an argument before it really began.

In the grips of inspiration, I decided it would be a Mad Hatter's Tea Party, complete with tea, tarts, croquet and costumes, but no presents, birthday cake or central actor.

Katy called her friends and I invited parents who I knew had enough child in them to enjoy this wonderland fete.

As an aficionado of yard sales and mother of a theatrical foursome, I have a large collection of dress-ups, so I draped the surplus over a ladder in the back yard for anyone who showed up costumeless or unexpected, lured by the crepe-paper streamers hanging from the trees.

After the initial shock, my husband entered into the spirit of the occasion, donning tuxedo, Winnebago T-shirt, Geraldine wig, sneakers and Mongolian army hat to greet the guests.

Leslye arrived first with her brood dressed, respectively, as Alice, the Queen of Hearts, a three-year-old foreshortened Mad Hatter and an out-of-context but costumed cowboy. Leslye, in a gold mini-skirt cocktail waitress outfit festooned with shamrocks and topped by a black aviator hat, outdid the lot of them.

Eric and Robert, visiting and at loose ends, wandered in and asked if they could stay. Sure, but they had to dress up. After fumbling through the jumble of possibilities, they both emerged looking like Alfalfa in drag. This didn't deter Rachel, however, who announced that she might want to marry Eric someday.

At this point, we freed the dog who had been wearing a witch's hat, but was so ashamed of his undogly appearance that he had dug a hole under a bush and hidden.

Since we were honoring no one in particular, the refreshments were communal and casual: peanut butter and jelly sandwiches, Koolaide, fruit and tarts. Chosen for their thematic value and ease of assembly, the tarts were concocted with ready-made graham-cracker shells, instant pudding and canned cherry-pie filling.

Muttering "I'm, late, I'm late," Betsy arrived sporting a black velvet doctor-of- chemistry cap and gown to match her army boots.

With the breeze twirling the propellers on her headgear and Koolaide glistening on her cut-off purple evening gown, my daughter led a rousing chorus of "A very unhappy birthday to you, to me . . ." Her father accompanied the group on his 50-cent Humanatone, an odd, plastic instrument played with one's nose, while my six-year-old son rolled his eyes in disbelief at the scene.

After tea, the kids set out the croquet wickets, banging them through, over and around each other with mad-hatter abandon. The flamingos were missing, but the Queen of Hearts would have been proud of the mallet-wielders' style.

Later, costumes were shed, boys wandered off to hunt worms, girls retired indoors to play with dolls and art supplies and adults settled into the time-honored party ritual of sip and chat.

When the last guest departed, Katy, warmed by the success of her tea party, asked, "When do we have Toby's unbirthday party, mommy?" I wasn't brave enough to tell her there are 364 unbirthdays in a year.