The day was hot, but a blissful contingent of 3,500 self-proclaimed disc worshipers convened yesterday in the name of Frisbee.

Everybody is here because Frisbee is fun," said Bill Good, assistant art curator at the Air and Space Museum, and the driving force behind the second annual Smithsonian Frisbee Festival sponsored by the National Air and Space Museum. "We wanted people to come and see what could be done with the disc and then teach them how to do it."

Which is exactly what the thousands of people did on the grounds below the Washington Monument. Spurred on by a gravity-defying exhibition from Krae Van Sickle - the 17-year-old redheaded 1978 World Frisbee Champion - and Jens and Erwin Ve'squez, who together were freestyle world champions in 1976 and 1977, Frisbee freaks eagerly lined up for pointers from a hundred instructors there to help out.

Many of the enthusiasts seemed to have been introduced to Frisbee early in life. Good, for instance, said he started out at age 6 or 7 with a disc called "Pluto's Platter" while champion Van Sickle, a high school student in Woodstock. Vt. who also happens to be first-rate skier, cut his Frisbee teeth in the middle of Manhattan.

"I learned to play Frisbee in Central Park", said Van Sickle, "when my father said he would not toss a football or baseball with me until I threw a Frisbee with him.

Little did Van Sickle dream, however, that tossing a Frisbee with dad would lead to big bucks. Or at least relatively big - given Frisbee's burgeoning stature in the sports world. Van Sickle picked up "between $4,000 and $5,000" for his August victory at The World Frisbee Championship held at the Rose Bowl in Pasadena, Calif. For that meet "Wham-O put up the $25,000 prize money shared among the winners. "There are currently 40 or 50 people who make a good living off of Frisbee right now," said one Wham-O company official at the gathering.

The Vela'squez brothers, natives of Peru who now live in New Jersey, have just returned from a seven-week tour of Europe sponsored by the Wham-O's European distributors their agenda included, among other things, a stadium exhibition appearance for 3,000 fans in Sweden. "The game is like a ballet," explained Jens, "it's a dance."

Frisbee tossing has surged to remarkable popularity in the last decade. In fact, what may have started in the 20s, as a collegiate pastime, tossing a pieplat back and forth - has today turned into an event that may be destined for the Olympics.

Frisbees - as legend has it - first appeared at Yale (some say Princeton) when a few students began scaling the plates, clearing their fairways by yelling "Frisbee" much in the same way that golfers yeli "fore."

However, it wasn't until 1954 that Wham-O Mfg. Co. then a manufacturer of sling shots, blowguns and crossbows, decided that what this country needed was a flying saucer. So they produced one - calling it Pluto's Platter.

That disc, however, sat on the shelf virtually unknown to potential fans who at that time preferred to twist away their time with the company's Hula Hoop, also an original Wham-O toy. (Although the company over produced and ended up with plants full of hulahoops, it still sells 1.8 million of them a year, and holds Hula Hoop championships in almost a hundred cities.

Frisbee finally came to its own in the late '60s because, as one participant put it yesterday, "young people were looking for a means of self expression and Frisbees - like jogging - allows you that kind of individually." The result, for Wham-O, at least, is that of their annual $20 million in sales, 25 percent comes from Frisbees. Of course, with all those people expressing themselves, yesterday, there were some casualties, as thousands of colorful projectiles shot from hand to hand. Five-year-old Gabrielle Wiess was one of the early drop-outs in "The Frisbee Wars" as her father called them. Nonetheless the slight blow to Gabrielle's head had no determent effect whatsoever on her 3-year-old sister Miranda who valiantly continued hoisting her own Frisbee.

Some people look upon Frisbee as a way man can get closer to man's best friend. Eldon McIntire, for instance, is the owner and trainer of "Leaping Luke," an Australian sheepdog who yesterday dazzled the crowd by retrieving Frisbees with his Baryshnikov-like leaps.

"Your dog is a natural frisbee player," explained McIntire. "People are sharing an activity with their dog. It takes about a year to train them so that when you're ready to play, they can be your partner."

However, like all sporting events, Frisbee is not without its groupies. Yesterday's gathering was dotted with pretty girls in short shorts, many of whom may have been feeling like one young woman who toted a Frisbee prominently inscribed with her name and her telephone number.

"I always believe in being prepared," she explained. "One toss aimed at the right feet and . . . who knows?