If they tell you Rare Essence will be jamming at the zoo today, don't believe it. The annual rumor is merely part of a body of Washington folklore that has grown around Easter Monday at the National Zoo, one of the busiest days of the year.

"For about the last four or five years, we have had a nemesis somewhere who drops this rumor that there are rock concerts at the zoo," says Samuel L. Middleton Jr., who heads the zoo's Office of Police and Safety. "That's never the case. We would never have a rock concert, and never on an Easter Monday, I know."

The Easter Monday visit has been a tradition for many Washingtonians, especially blacks, almost since the zoo opened in 1890. Since the turn of the century, says National Zoo historian Billie Hamlet, people have come by foot, streetcar, cab, automobile and Metro to picnic, roll Easter eggs on the zoo's Lion House Hill and enjoy the holiday.

Why Easter Monday, rather than Saturday or Sunday?

In those days, when many Washington blacks worked as domestics, "the servants were given Easter Monday off," Hamlet says, "and they selected Lion House Hill to roll eggs."

The visits evolved into major family expeditions and high-fashion Easter parades.

Louise Hutchinson, director of research of the Anacostia Neighborhood Museum, remembers her childhood Easter Monday zoo visits "with warmth and happiness," when her father would take her and her eight siblings and lay out the basket and blanket at his favorite site -- close to the ducks.

The holiday excursion rates on the old streetcar system may have been a contributing factor to the tradition, Hutchinson says: "Many poor families didn't have their own automobiles . . . And children could ride free on Sundays and holidays with their parents."

She remembers "the joy of preparing for the trip, painting the eggs . . . You'd have your eggs, your little Easter bunny, your little chocolate rabbit. And you can't omit that picnic basket. Our mother would fix our favorite sandwiches -- peanut butter and jelly sandwiches, bologna sandwiches, egg salad sandwiches. And there certainly would be lemonade. And home-baked cookies."

Washington residents "didn't have many places to go to in those days," says District native Lillian Rogers Parks, 89, who visited the zoo yearly during her teens with her family and later "when I was old enough to have a friend take us in the car. We'd go driving and go to the zoo . . . Visitors could stay all day, they didn't have to come home till the evening. That's why many went there . . .

"But you have more places to go to now."

Many families still visit the zoo on Easter Monday, but over the years, particularly in the 1960s, the crowds moved more to teen-agers and young adults, who at times became unruly. While zoo staffers say most of the problem days are long past, Easter Monday is still a day to beef up the ranks, to contend with the crowd and expect just about anything.

Just when and where the concert rumor began, nobody seems to know. But it has been so persistent, zoo officials have had to take measures to spike it.

"One year we had a group come in who insisted they heard it on the radio . . . Some disc jockey saying, 'Let's all go out to the zoo for the concert,' " says zoo spokesman Bob Hoage. "The next year we sent out public service announcements to all the radio stations saying there was no concert." But the rumors and the crowds have persisted, for reasons no one seems able to explain.

"We routinely increase our staff 25 percent," says Middleton. As a 23-year veteran of the force, he has seen all kinds of Easter Monday incidents -- teen-agers looking for a rumored funk concert being merely the latest.

Take the Gaboon viper incident two years ago, when 16-year-old Lewis Morton smashed a glass window in the reptile house, put two of the most poisonous snakes in the world in a plastic garbage bag and carried them away on a Connecticut Avenue bus. The python-sized vipers, with saucer-sized heads and enormous fangs, bit him through the bag as he left the bus. It took serum jetted from Africa to save him from certain death.

Or take the isolated stonings of birds and taunting of animals; or a dark period in the 1960s when roving street gangs annually made the zoo part of their Easter Monday schedule ("Folks got drunk," says Hamlet), forcing keepers to close the animal houses that day for a time. Those crowds, he says, "obviously didn't come to see animals."

But the troubleshooting has been less necessary lately, says William Xanten, collections manager at the zoo, due in part to the bad Easter weather in recent years. Plus, says Middleton, "Most of the former gang members have grown to be adults."

"There have been instances" of vandalism or unruly behavior in the past, says zoo spokesman Hoage, "but in recent years it's not been that much of a problem." In addition to the police force, more zoo staffers (particularly animal keepers) will be out today, Hoage says, to "be more available to the public for questions."

The National Weather Service predicted good weather for today, with high temperatures around the mid-70s. And, as any good zoo staffer will tell you, when the going gets sunny, the zoo gets crowded.