THE MEANING of Thanksgiving and the manner of its celebration have changed little since the days of the Pilgrims," says The Encyclopedia Brittanica. "Thanksgiving was and is basically a home festival with religious overtones."

Wrong.

On Thanksgiving, Americans watch televison.

Viewing is so abnormally high that rating-service computers need a special program to digest the data. Normal weekday viewing levels double by mid-morning -- when, according to Nielsen, about 45 percent of all homes in the United States have their television on.

A full one-third of America's households tune to afternoon football games, and each house has 25 percent more viewers than on a normal Sunday afeternoon. By comparsion, viewing drops about 20 percent on Christmas Eve, and approximately 4 percent on Christmas Day.

"Other holidays are the pits," says a Nielsen spokesman. "But Thanksgiving is a gathering of the clan."

As a result, every hour of Thanksgiving programming is carefully planned with the ratings in mind; and the traditional scramble for audience dollars influences not only the specials, but determines which teams play in the football games -- and even which floats appear in the parade.

Most experts agree that the day after Thanksgiving is the highest volume retail business day of the year. "There's the Macy's parade on television, and people are thinking about shopping," says a spokeswoman for the National Retail Merchants Association.

Nielsen figures show that over 70 million Americans tune in the Macy's Thanksgiving morning parade and that for every 1,000 households, there are 865 women, 623 men, 146 teenagers and 544 children watching. Thanksgiving afternoon, football games draw 524 women, 1,174 men, 131 teen-agers and 174 children per 1,000 households.

Commercials follow these demographics, NBC, for example, reports that most Macy's parade commercials are for "toys, soft drinks and fast foods."

A 30-second parade commercial sells for $22,000, twice the usual rate for that time period. During the ball games, 30-second spots sell for $85,000, compared to $12,000 customary for weekday afternoons.

Most programming changes for Thanksgiving are calculated not to recall the holiday, but to take advantage of advertising opportunities. This year, network schedules show only one Thanksgiving-theme program, CBS' "Mayflower: The Pilgrims' Adventure." CBS Vice President Bernie Sofronski says, "We make a positive statement in terms of our heritage. It's totally bases on research. We even know how many pigs and chickens came on board the Mayflower."

Other special programming plays up to the youthful, family-oriented audience. CBS kicked off this year's Thanksgiving season with "Bugs Bunny Thanksgiving Diet" last Thursday, to be followed this week by "A Charlie Brown Thanksgiving," "Raggedy Ann and Andy" and "Young Love, First Love." NBC warms up with a "Bee Gees Special," and ABC offers a special episode of "Family."

But the networks put little money andy effort into Thanksgiving evening because viewing levels decline slightly. "The television has been on all day and people are tired," hypothesizes one network official. And series offerings largely ignore the holiday: "There may be a turkey dinner on some show, but that's about all we do," says an NBC spokesman. The family audience prompts no conscious network effort to tone down sex or violence.

Audience levels are back above normal by 11 p.m. Thursday and remain healthy through Friday and Friday evening. the networks respond with more specials, again oriented to the younger audience. This year, ABC presents "Birth of the Beatles Special" and "Roller Disco Derby," and CBS weigh in with "Captain America," based on the comic-strip character.

The search for ratings -- and thus more advertiser dollars -- also dominates the selection of football contestants.

"It was just a gimmick," says a Detroit Lions spokesman explaning why the Lions played the first professional football Thanksgiving game back in 1934. "The team had just transferred from Portsmouth, Ohio, and the new owner wanted to get 'em going."

In 1951, CBS braodcast the Detroit game nationally and made the gimmick a television tradition. When the winning Lions of the early 1950s faded, broadcasters responded with a new tradition: The Thanksgiving NFL doubleheader starring Detroit in the first game and the perennially popular Dallas Cowboys in the second. Dallas has played every Thanksgiving, except two, since 1966. "It's a matter of us being able to generate the best ratings for them," says a Cowboy spokesman.

Similarly, the ratings have bounced college contests around. A National Collegiate Athletic Association game used to be played on Thanksgiving Day, but the pros attracted too many television viewers so that NCAA tried Thursday evening, "That didn't work," says the NCAA's Tom Hansen. "Football games were on all Thursday afternoon. Mother had been in the kitchen all day working. When she took her apron off and sat in front of the television, she was in charge -- and she was not about to listen to another football game."

So the college games moved to Friday afternoon. (Friday evening was impossible because ABC and the NCAA contract, is first in the ratings, and its prime time iw worth too much.)

In addition to football, networks love a parade.

For the past 18 years, CBS has braodcast the "All American Thanksgiving Day Parade," a three-hour live-taped mixture of Macy's parade in New York and others in Toronto, Honolulu, Detroit and Philadelphia. During the same nine-to-noon time slot, NBC has presented the Macy's parade live for 26 Thanksgivings.

Aware of their tremendous audience and sales appeal, the networks have turned these parades into endless commercials for their own programs. This year's CBS parade host is actor Bill Conrad, who has a "Cannon Revisited" special coming up on the network (Conrad has hosted the parade when he had no show on the air). And, each city has its own CBS hosts, stars of CBS programs -- "Dallas" and "Dukes of Hazard" in New York; "Lou Grant" and "M*A*S*H" in Philadelphia; "Waltons" and "Trapper John, M.D." in Detroit; and "WKRP" in Toronto; and, of course, "Hawaii five-o" in Honolulu.

"All American Parade" Executive Director Mike Gargiulo says no one makes program plugs. "None of their shows is in trouble," he says. "It's a personal benefit. They do it because they can be themselves. Selling their shows is not the intent at all."

Over on NBC, the "Tonight Show's" Ed McMahon has hosted their Macy's parade coverage for the past four years. Joining McMahon are a variety of television personalities and would-be personalities with no known connection to Thanksgiving. Last year, in a typical example, Macy's promotional material boasted that "teen-age Kathy Kurtzman who plays Heidi in the forthcoming TV musical" would be on hand.

Parade planners, in turn, have learned that the most popular parade to watch on television is one involving television. Macy's 1978 floats, for example, included no Pilgrims or Indians. But they did have Bullwinkle the Moose, Kermit the Frog, Underdog and a 24-foot replica of Sesame Street. This year's Macy's parade will include a television camera float, the Muppets and yet another Sesame Street display.

Most children now consider these parades a mandatory part of the holiday. In fact, according to the Census Bureau, 58 percent of all Americans have been born since World War II, which means they have, at best, dim memories of Thanksgiving without television. And from the advertisers' point of view, that's all gravy.