Undated (AP) _ Americans planned parades, food fests and fireworks for the Fourth of July weekend, but the nation's worst drought in decades put a damper on many pyrotechnic displays because of fears of wildfires.

And in Miami, authorities issued a weekend warning as millions of mosquitoes swept into the area from the Everglades.

Organizers in St. Louis expected more than 3 million visitors for the annual VP Fair, billed as ''America's largest birthday party.''

Nine stages scattered across the St. Louis Arch grounds will be used by entertainers ranging from Glen Campbell to Chinese acrobats. Vice President George Bush is scheduled to speak at naturalization ceremonies for 300 people.

The three-day fair also includes today's annual Veiled Prophet Parade - staged by a secret social group, the Veiled Prophet Organization - airshows, a hot air ''Balloonfest,'' an Olympic-qualifying bicycle race, a steamboat race, ethnic food and nightly fireworks displays.

But in other parts of the Midwest and elsewhere, Independence Day revelers will have to find alternatives to fireworks due to bans spurred by the dry weather.

On Tuesday, Wisconsin Gov. Tommy G. Thompson signed an executive order banning private fireworks displays - even sparklers. But local governments may make exceptions where conditions allow, and community fireworks will be permitted as long as there is adequate fire protection.

Counties and communities in Iowa, North Dakota, Kentucky, California and elsewhere also banned or restricted fireworks out of fear of setting grass and forest fires.

''The potential is there for a disaster this year,'' said Les Parr, a Riverside County, Calif., fire prevention officer. ''The brush is dry and well-cured and the danger and potential for fires is a lot greater.''

But several Kansas communities that had banned fireworks rescinded the bans after rain fell Thursday, easing the drought.

Independence, Mo., came up with a novel way of celebrating the Fourth of July during the drought. The birthplace of President Harry S. Truman will replace its traditional fireworks with a laser light show.

After Philadelphia Mayor W. Wilson Goode announced in May that his city's four-day Freedom Festival would be cut to only a few events on one day, a number of businesses and organizations donated money and other resources to help the city throw a weekend bash after all.

The scaled-down festival starts today with the fifth annual July Fourth Restaurant Festival, where chefs from 21 restaurants will serve their specialties along the Delaware River.

In Cape Canaveral, Fla., space agency workers will hold a special Fourth of July ceremony when they move Discovery to the launch pad to be readied for the first space shuttle mission since the Challenger tragedy.

Dave Hilmers, one of the five astronauts slated to fly the early September mission, and NASA officials will watch from a flag-bedecked grandstand as the spaceship is rolled out of an assembly building at 12:01 a.m. Monday.

In his Fourth of July message, President Reagan said each celebration of the holiday stirs ''the pure patriotism of childhood.''

''With each flag, with each parade and picnic and burst of fireworks, we can't help but recall the first stirrings of our deep love for America,'' the president said. ''No matter how many Fourths we Americans have seen, every new one revives in our hearts the pure patriotism of childhood.''

In Florida, rain and damp weather caused swarms of mosquitoes to breed and southwest winds, unusual for this time of year, have blown them from the Everglades by the millions into populated areas.

Officials in the Miami area issued a mosquito emergency and warned weekend revelers to arm themselves with repellent.

''In many, many parts of Dade County we're experiencing landing counts on human beings of 50 per minute,'' said Pat O'Conner, Dade County's assistant director of public works. ''That is, you walk out in your back yard and you have 50 mosquitoes on you in one minute.''