ON Oct. 6, 1683, 13 Mennonite families who had sailed from Krefeld, Germany, in search of religious freedom, landed near what is now Philadelphia and founded Germantown, Pa. During the next three centuries they were followed by seven million other German immigrants. Today more American citizens--28 percent or more than 60 million living descendants--can trace their roots to Germany than to any other country.

To help all Americans to be more aware of the significant cultural, commercial and political contributions made by German-Americans, more than 500 events have been planned in 43 states and the District of Columbia as part of the Tricentennial Year of German Settlement in the United States. West Germany is also observing the anniversary.

Activities already underway include lectures, dramatic and musical performances, exhibitions, seminars and scholarly conferences. Among the observances are "American Summer, '83" in Berlin featuring cultural events; exhibitions in New York and Chicago on "300 Years of German-American Relations"; and special events in Los Angeles, Denver, Dallas, Milwaukee, Atlanta and other U.S. cities.

In Baltimore, the 83rd Annual German Festival with exhibits, crafts, food, beer tents and continuous live music will run from Aug. 19-21 at Rash Field, Inner Harbor. The city's German community will host many activities as part of "International Week" Oct. 22-29. In Frederick, a "Bavarian Fall Festival" will be held Oct. 15-16 on the Frederick County Fair Grounds, featuring German-related crafts, music, dancing and food.

Philadelphia, of course, will be the focus of a number of observances, including a "Summer Fest" with a typical German country fair atmosphere July 6-7 at Penn's Landing; and a "Tricentennial Conference of German-American History, Politics and Culture" Oct. 3-6 at the Annenberg Center, University of Pennsylvania.

The District is also the scene of many events, such as a "Tour of German-American Washington," which includes historic churches and buildings, the Folger Library and Library of Congress, monuments to Martin Luther and Baron von Steuben, and the Federal German Republic Embassy, on Oct. 23; and "Martin Luther Jubilee Week" from Nov. 6 to 12.

For a free list of scheduled events in each participating city, indicating sponsors and giving telephone contacts for further information, write to the Presidential Commission for the German-American Tricentennial, 730 Jackson Pl. NW, Washington, D.C. 20006. DISNEY WORLD SUMMER: Epcot Center and the Magic Kingdom at Walt Disney World, Lake Buena Vista, Fla., continue to nourish the tourism boom in Central Florida. Between 15,000 and 20,000 additional hotel rooms under construction in the nearby Orlando area are expected to be ready for occupancy this fall.

Extended summer hours--9 a.m. to midnight--are now in effect through Aug. 27 at both Disney attractions. New parades and fireworks displays will be featured, along with a musical show, "Show Biz Is," which premieres in the Magic Kingdom next month. It's a song and dance production of Broadway, Hollywood and circus themes on the Tomorrowland Theater stage.

Disney officials say the busiest days at both Epcot and Mickey Mouse's playground are Monday through Wednesday; Sunday's usually the slowest. And don't be misled by the building boom--if you want to be sure of a hotel room, make your vacation reservations well in advance.

Central Reservations Office, P.O. Box 78, Lake Buena Vista, Fla. 32830, phone (305) 824-8000; Walt Disney World Information, P.O. Box 40, Lake Buena Vista, Fla. 32830, phone (305) 824-4521. 400th ANNIVERSARY: Newfoundland, the world's seventh largest island and one of Canada's popular Maritime Provinces, is celebrating the anniversary of its Aug. 3, 1583 founding as the first British colony in America--and that's good news for U.S. tourists and Canadians.

Activities connected with the observance, which begins officially this month with the visit of Prince Charles and Princess Diana, are scheduled throughout the province through October. They are occurring at a time of rising Canadian hopes that the country's tourist trade, hurt by last year's recession, will rebound along with the U.S. economy. In 1982, 33.3 million of the 34.8 million foreign visitors to Canada were Americans, and they spent $2.4 billion. The U.S. dollar is now worth about 20 percent more than the Canadian dollar.

Our northern neighbors will offer a warm welcome. In Newfoundland, aside from the perennial opportunities to fish, hunt, camp, hike and take scenic drives, there will be music festivals, folk dance performances and a chance to hear the Royal Canadian Mounted Police Concert Band which will be on tour.

More information on Canada from the Canadian Government Office of Tourism, Suite 200, 1771 N St. NW, Washington, D.C. 20036, phone 223-2855. AFFORDABLE SWEDEN: Not long ago Sweden was considered one of the world's most expensive tourist destinations, but that's no longer true says the Swedish Tourist Board.

A healthy dollar, depreciations of the Krona, and tax-free shopping for foreign visitors are among the reasons why the purchasing power of American travelers in Sweden has increased "by more than 50 percent" over the last two years. That will be especially pleasing to the more than 12 million Americans of Scandinavian descent who constitute a strong ethnic market.

Non-Scandinavian tourists receive average refunds on their purchases of 15 percent. Other money-savers are "Stockholmskortet," a discount card in three versions costing from $7 to $14 and providing from 24 to 72 hours of free travel on the city's public transportation, several free tours, admission to more than 50 attractions and other benefits; a reduced rate card allowing deductions of 45 percent on train ticket prices within Sweden; a Scandinavian Rail Pass good for 21 days of low-cost travel throughout the region; and a reduced price tourist menu in more than 400 Swedish restaurants--a complete meal for $5--until Sept. 1.

In addition, Scandinavian Airlines has just inaugurated "the fastest air service ever between the United States and Sweden"--7 1/2 hours--with twice-weekly non-stop flights between New York and Stockholm (it's the only non-stop service). In connection with its new flights, SAS and three tour operators are offering new one-week packages to Stockholm with prices starting at $775 per person, double, including air fare from Manhattan.

For information: SAS, 138-02 Queens Blvd., Jamaica, N.Y. 11435, phone (212) 657-2575; or Swedish Tourist Board, 75 Rockefeller Plaza, New York, N.Y. 10019, phone (212) 582-2802.