Flag Day has never really caught on -- it's always been a stepchild of American holidays. It's no secret, however, that patriotism and flag-waving are in vogue.

"There's a lot more to it," says Whitney Smith, director of the Flag Research Center, Winchester, Mass., "than waving the American flag. It truly is a symbol of freedom."

But it is also a symbol of profit.

Flags are big business. Americans love to fly flags of all sorts: holiday flags, religious flags, family flags . . . Because major flag companies are privately held, however, they can be -- and are -- tightlipped about how much they make: Sales figures are secret. But local flag companies admit that business is booming.

Claude Haynes, president of National Capital Flag Co. Inc. of Alexandria, says burgeoning orders "from all 50 states, Guam and overseas" helped move his business from its modest beginnings "on the dining room table" in 1962 to its present 20,000-square foot facility employing 23 people. "We're running out of space and I'm looking for two more full-time seamstresses."

Haynes and other area flag suppliers agree that most of the orders come from businesses and government agencies -- local, national and foreign. "Other than holidays," says Erica McCall, soon-to-be president of the 123-year-old Copeland Flag Co. of Alexandria, "the general public doesn't really bother to buy American flags." (The offices of U.S. congressmen, on the other hand, are flooded every year with thousands of citizen requests for "flags flown over the Capitol.")

People increasingly are looking for a means of self-expression in a society that is becoming more and more depersonalized. "A flag," asserts Smith, "is a wonderful way to do it. It's colorful and it draws attention."

Smith, who calls himself America's only full-time vexillologist, notes that custom flags don't come cheap. In consultation with the customer, and for a fee beginning at around $300, Smith and his colleagues will come up with an appropriate design, "a unique, professional-quality piece of artwork in full color, suitable for framing and for use by a flag-making firm. The design could be copyrighted if the person wanted to protect it legally."

Several of the local flag companies say they also will help design and make custom flags. Says Haynes: "We'll make any flag, as long as it's not illegal, immoral or indecent."

Many of the people who get flags also buy the poles from which to fly them. Haynes and Jarvis McCall, Copeland's pole expert, say that their firms supply poles in numerous lengths -- 10 to 70 feet and up -- and in several grades. A 20-foot aluminum pole might go for as little as $130, where a 100-foot steel pole could run $15,000, plus labor.

Homeowners easily can install some of the lighter, shorter poles, but the experts say installation of the heavier, longer poles should be left to them. In all cases, local zoning authorities should be consulted for any restrictions on flags and flagpoles.

A rule of thumb for matching flag and pole: The above-ground length of the pole should be four times the length of the flag. Those who don't want to go the flagpole route can purchase kits containing a 3-by-5-foot flag and 6-foot pole for mounting on the side of a house for as little as $10 or $20.

Whatever the size, says Smith, "There's something about the motion of a flag and its colors that is very appealing. It's an artificial object, and yet it has a life of its own, almost like a tree. Flags have a very strong emotional pull. And," he adds, "generation after generation of Americans have chosen to fly the American flag and honor what that represents."

President Reagan will lead today's honors with a scheduled visit to Baltimore for ceremonies at Fort McHenry, original home of the Star-Spangled Banner, and many Americans attending similar ceremonies will pause at 7 o'clock EDT tonight to pledge their allegiance.