To the strains of familiar Christmas melodies, about three dozen travel writers from around the country checked in at the Hyatt Regency on Capital Hill on Friday night for a weekend of "Christmas in August."

The two-day funfest, sponsored by the 45-member Hotel Association of Washington, was designed to promote the city as more than just a prime tourist attraction. Emphasis was placed on the International aspects of the nation's caital, and the subtheme for the entire production centered on the countless number of absolutely free-of-charge attractions and cultural events.

"Almost everything worth seeing is free," said Edward McMillian, partner and managing director of the International Inn and president of the Hoel Association.

Mayor Marion Barry addressed the travel writers at luncheon Saturday, taking the opportunity to plug the city and the voting amendment.

On his favorite topic, Barry notified his visitors they had come to "the last colony . . . we have no votes for the Senate or House of Representatives," before beseeching the assemblage to "see the other Washington, the local city -- as diverse and beautiful as any in the country."

Moving on to jobs and commerce, Barry praised the tourist and hotel industries for their impact on local commerce and job availability.

"Tourism is our largest and fastest growing industry," Barry remarked, "bringing 30,000 new visitors to Washington daily and providing upwards of 100,000 jobs." Total dollar volume of the tourist trade, according to the mayor, approaches $2 billion annually.

"The new convention center scheduled to open in January, 1983, will bring in another 300,000 visitors a year," Barry predicted.

Yesterday the party ended with brunch at the Sheraton Carlton. The goals of the Hotel Association, primarily to create a little good press for the city, seemed to be achieved. A guest from the American Automobile Association asserted that the city is "within economic striking distance" of the entire east coast, while writers from the New York-based Travel Agent magazine marveled that "there's no trash or empty soda bottles on the streets."

But Mayor Barry summed up the real meaning and intent of the whole weekend during his speech Saturday. He called a stay in the city "enriching and entertaining," and said, "incidentally, while you're here spend as much money as you can. We need it.