Long before the 10 a.m. opening time Monday, the children will slip into the Maury Elementary School playground to check the preparations for what is approximately the 33rd annual July Fourth Revolutionary Picnic in Alexandria's Rosemont neighborhood.

For this cozy corner of Washington suburbia, the fact that the event is not precisely on July 4 is immaterial. Rosemont people like to sit on their porches and forget about time. This Independence Day celebration encourages such indolence.

There will be some competitive events. Frisbee tossers will be asked to go the distance. Watermelon seed spitters will show just how explosive their tongue and mouth muscles can be. The bicycle decorators will stretch their creative impulses to the limits of red, white and blue, or whatever else works. The parents of the baby contest entrants will try to time their infants' naps for maximum alertness and cuteness at 11 a.m., the scheduled time for judging.

But veterans of the Rosemont celebration know there will be few, if any, disappointments. "They have all kinds of contests, but everybody wins," said longtime neighborhood resident Bill Masterson.

This year's announcement defines the soul of the event in a series of terse suggestions: "Decorate your bike. Dress your dog up like Betsy Ross. Catch a water balloon. Throw an egg. Dunk a friend. Jump in the sack. Freeze your brain. Eat a frankfurter. Sing a song. Salute the flag."

"It's mostly for the kids," said Judy Miller, real estate agent and president of the Rosemont Citizens Association, which coordinates the celebration. "It's family oriented."

Rosemont--northwest of Old Town not far from the towering hulk of the Masonic Temple, which looms over the King Street Metro stop--is a tidy patchwork of brick and wood-framed houses, running the spectrum of American suburban architecture from farmhouses to bungalows to cottages to much-modified structures that defy easy identification. Most of the lots and houses are small but valued for their location, their shade-yielding maples and firs, and their spiritual cohesiveness. Few other communities still celebrate the holiday with such old-fashioned, low-tech enthusiasm.

The picnic announcement says "patriots, a juggler and city council members" will be in attendance, not making clear if those are three distinct categories of people. Miller said she expects 300 to 400 participants, depending on the weather.

All the festivities take place on the Maury Elementary School playground, an expanse of asphalt basketball courts and a baseball field with at least as many bare brown spots as grass. There is never actually a parade on the Fourth in Rosemont, although the bikes are wheeled around in line for all to admire. Entrants in the pet show attempt to move around the baseball infield.

One veteran pet show judge, who asked not to be identified for fear of compromising her judicial integrity, said the trick in awarding ribbons--all blue--is to decide the proper categories of achievement.

"Usually they will have a parakeet and an iguana and some homeless cat and a bunch of dogs," the judge said. "So we will give a blue ribbon for the longest tail, or the nicest personality, or maybe the largest ferret."

"Some of these kids know when they are being patronized," she added. "They will say, 'What do you mean, largest ferret?' " But within a few minutes, they notice the cotton candy line and recover their holiday mood.

Politicians will give very short addresses, and the crowd will sing patriotic songs. Frederick, a juggling clown, is expected, as well as firetrucks and police cruisers, for youngsters to indulge their fantasies.

There also will be a dunk tank. Years ago, some politicians were dunked. But for the victims, "that grew old very fast," Miller said. Now children dunk other children. All are refreshed, and content.

The bicycle-decorating contest will be at 11 a.m., promising both two- and three-wheel categories.

This year's Rosemont celebration will be diminished by the lack of active involvement of the Clausen family, Lisa and Jim and their three children. In years past, the Clausens have had their own pre-celebration breakfast party for neighbors, with bagels and coffee and watermelon and lots of colored ribbons, streamers and other essentials for bicycle decoration.

"I didn't realize they weren't having it until the fifth and I have a child going off to camp that day," Lisa Clausen said. She and her two remaining children may stop by for hot dogs and snow cones, but organizers said it wouldn't be the same.

The grand finale will be the water balloon and egg tosses, scheduled at 2 p.m. so that contestants who find their clothes and bodies besmirched can go home to change without missing any of the other events.

The Independence Day celebration in Rosemont has gone through many phases since it was, according to legend, first organized by residents Betty Murphy and Owen Malone in 1966. Miller said the essential small-town favor has survived, even while the entertainment has changed.

It has been a long time, she said, since she has been able to enjoy the event as a participant rather than an organizer. But she does remember with pleasure a ladder climb that tempted participants to think they could get to the top until it suddenly swiveled and brought them back down again.

There were yelps of surprise and frequent pledges that the ladder could be beaten, if the contestant had just one more try. "That was a lot of fun," Miller said. She said she would like to find it and bring it back again.

CAPTION: Riley McDonald, left, Deva Solomon and Bud McDonald line up for a sack race in 1992. Rosemont has held its holiday event for more than 30 years.