Dutch champion Ajax visits RFK stadium for a friendly with D.C. United on Sunday, beginning a summer when Manchester United, FC Barcelona, Real Madrid and the Spanish national team play in the U.S. (BAS CZERWINSKI/Associated Press)

The U.S. soccer season officially began in the middle of March. The unofficial season starts this weekend.

With Dutch champion Ajax’s arrival in Washington for Sunday’s friendly against D.C. United at RFK Stadium, European clubs have begun their annual summer retreats to the United States and Canada for matches, training sessions, sightseeing and marketing opportunities.

The sport’s biggest brands – Manchester United, FC Barcelona and Real Madrid – are on their way. So are England’s Manchester City, Everton, Newcastle and West Brom, as well as Mexico’s most popular teams, Chivas and Club America.

The influx isn’t limited to club teams. Spain, which lifted the World Cup trophy last year in South Africa, will play the U.S. national team in two weeks in Foxborough, Mass. And the CONCACAF Gold Cup, a biennial tournament that crowns the champion of the North and Central America and Caribbean region, returns next month to 13 U.S. venues, including RFK for a June 19 quarterfinal doubleheader.

Visiting clubs will play not only MLS teams – D.C. United hosts Everton on July 23 – but one another as well. The most anticipated club event is July 30 at FedEx Field in Landover: Barcelona vs. Manchester United in a rematch of next weekend’s UEFA Champions League final at Wembley Stadium.

Almost every seat at the Washington Redskins’ home, priced between $40 and $550, was gone within two weeks. About 30,000 tickets were claimed in a pre-sale for Redskins season ticket holders and other groups, team executive Mitch Gershman said. The common pre-sale count for non-NFL events at FedEx, such as college football games and concerts, is 20,000.

While Ajax arrived days after winning a 30th league title, most will turn up in July for the beginning of preseason training ahead of the August-to-May calendar. West Brom chose Santa Barbara, Calif., as a base.

For most teams, the impetus is business opportunity: strengthening brand names and collecting appearance fees.

Manchester United is expected to play in front of full houses at NFL stadiums in Foxborough, Chicago and Landover. The English champion will also serve as opponent for the MLS all-stars July 27 at 25,000-seat Red Bull Arena in Harrison, N.J. Besides FedEx Field, Barcelona will appear in Miami and Dallas, while archrival Real Madrid will play in Los Angeles, San Diego and Philadelphia.

Many of the friendlies are part of the World Football Challenge, developed by MLS’s marketing arm, Soccer United Marketing, and Creative Artists Agency, an entertainment powerhouse. MLS teams involved benefit from the increased exposure, both in their own markets and globally.

D.C. United has tested itself against foreign clubs since its inception in 1996. The most notable were Real Madrid (in Landover and Seattle); England’s Tottenham Hotspur (at RFK and in London) and Chelsea (in Landover); Argentina’s Boca Juniors; and Italy’s AC Milan (both at RFK).

“It gives us a lot of credibility with our fans and it’s good for our players,” United President Kevin Payne said. “It gives everyone reason to believe we can compete with these teams on a given day and let’s us measure ourselves against established clubs.”

Even though the Redskins own the rights to the July 30 spectacle, D.C. United gained ties to the match by acquiring a large block of seats to include in its season ticket package. With some in its cache remaining, United’s prorated season ticket offer is, in essence, the only remaining avenue for Barcelona-Manchester United seats at face value.

United isn’t expecting a big crowd for the Ajax match (maybe 15,000). Despite a decorated history — four European championships and an acclaimed youth development program — the 111-year-old club doesn’t enjoy the international following of Manchester United or Barcelona or a superstar such as Wayne Rooney or Lionel Messi.

D.C. United, preparing for two West Coast league matches, will use a mix of regulars and reserves while resting ailing starters, including forwards Josh Wolff and Charlie Davies. On vacation after championship celebrations in Amsterdam, short-handed Ajax will undoubtedly take a casual approach.

“It doesn’t matter who it is, your competitive nature is always there,” United midfielder-forward Santino Quaranta said. “But these games are really about celebrating the history of our club and the teams that come over.

“And it’s good for the young guys — they get to trade jerseys” with a player from a famous team.