For thousands of Northern Virginia families, the Thanksgiving holidays have come to be as full of soccer balls as they are of turkeys.

Arlington will host its fifth annual soccer tournament starting tomorrow, with 1,800 players on 106 teams converging on the county for three days of competition, plus sightseeing trips squeezed in between day and night games.

In Reston, more than 2,100 players on 120 teams are expected to arrive tomorrow for the third annual soccer festival (unlike tournaments, the festivals have no "winners" or trophies). Sixty teams will come from out of state, including eight from Canada and two from Ireland, which also is sending a bagpiper.

Many of the Washington area's 35,000 soccer-playing boys and girls, aged 6 to 19, their coaches and families will attend other soccer tournaments, such as the Tidewater tournament held simultaneously in Williamsburg, Norforlk and nearby towns, and other, out-of-state tourneys.

Still others will take a brief break from what has become an almost year-round sport here, more popular than all other area sports combined. Soccer is the only sport with spring and fall seasons and hundreds of "traveling teams" that play in local leagues, and then on holidays and in summer go to tournaments around the United States, Canada and Europe.

Virtually the only holiday of the year still without an area soccer tournament is Christmas, because the ground usually is frozen. But over the New year's holiday several area teams travel South to play in events such as Miami's Junior Orange Bowl of soccer, where two Reston teams won last year.

Bowie, Springfield and the Braddock Road Soccer Association hold annual tournaments over the Memorial Day holiday, McLean hosts one in June to commemorate the end of school and the Journal newspapers sponsor a tourney over Labor Day. Last month, on Columbus Day weekend, a girl's tournaments with about 140 teams and 2,500 players was held on playing fields all over Northern Virginia.

But by far the year's biggest fests, with almost 4,000 participants, are the Reston and Arlington Thanksgiving celebration of soccer.

Organizing and running them has become so complicated that both Reston and Arlington now have soccer headquarters in local motels, with telephones and CB radio base stations. The major problems are lost referees and players, unable to find a particular soccer field in some remote section of the county.

The major costs of the tourneys are about $5,000 for referees' fees for the nearly 500 games to be played by Sunday, and for trophies and patches given to players. Visiting players also trade team patches with local players. Participating teams pay entrance fees of $50 in Reston and $75 to $100 in Arlington, programs are sold at games and Reston this year has a festival sponsor, American youth Hostels, which pays some of the festival costs.

The highest cost for families is transportation, which can be considerable for teams such as those from Dublin who have flown over for the Reston festival and a week of exhibition games and visits to the capital. Most players and coaches stay with families of host teams to cut down expenses, favors that are returned when local teams go on tours themselves.

Hosts are not hard to find. The Reston Soccer Association was deluged with offers to house the Irish players and with complaints from some families when they only got one player to house for the week, said James Falk, a Reston volunteer soccer director. Falk, who helped arrange what is now the second annual visit by Dublin teams when he was in Dublin three years ago, teaches mathematics at George Washington University in his spare time. Falk's two teen-age sons are playing in the festival and his wife Jean, a registered nurse, is handling publicity.

A number of Reston families hosted a team from Taiwan two summers ago when they toured this country and still receive occasional letters in Chinese, said Falk. "None of the players spoke English but they were a lovely bunch of kids and it didn't matter."

Most soccer festivals and tournaments are by invitation only, but there are now so many thousands of soccer teams and so many hundreds of tournaments that is easy to wangle invitations. Two Arlington teams will play in the Reston festival and two will play in Williamsburg.

Arlington soccer has gotten so big, with more than 4,000 players, and so difficult to organize that its soccer association now has a full-time, $18,000-a-year director.

"We're not that big yet; Reston has only about 2,000 players," said Falk.

Despite the growing clout of more than 1,000 soccer families among the Reston population of 21,000, the town still has no permanent soccer fields and most of the festival games will be played in Herndon and surrounding communities. The Reston tournament, which had 56 teams in 1976 and 96 last year, "probably won't grow any larger because we've just about run out of fields," Falk said.

He attributes the phenomenal rise in the popularity of soccer in part to the game itself - "kids just like to run and kick at things" - but also to the democratic way it is run, with every child required to play half of every game in the so-called house leagues, the basic soccer leagues. Even on traveling teams, formerly called "select" teams - which better players from the house leagues try out for - most players play every game.

"It's also a social affair and you meet people from all varieties of life. One of the Irish coaches is a house painter and one of the Canadian coaches a banker," says Falk.

"Soccer is unique, and so are its travelling teams," says George Towner, an Arlington attorney and volunteer helping with Arlington tournament. Towner is also an assistant coach on the Arlington Arrows, his 13-year-old son's team.

But even enthusiasts like Towner, who is also chairman of the county Park and Recreation Commission, say that soccer can become an almost year-round family preoccupation. Not only are there the various soccer leagues - the house leagues, all-star, traveling clubs - plus the tournaments and traveling club tours, but many kids also play on public and private school teams as well.

"Some kids are now playing soccer six days a week . . . and their families haven't has a vacation in years," he said.

That said, Towner admits the Arrows hope to be invited next summer to an Ohio tournament held by the teams playing in Arlington meet. "And we're already saving for a tour of England in the summer of 1980."