The Washington Monument grounds have been the site of many gatherings. Starting next month, the sprawling space will be transformed into a movie palace.

For five Mondays, movie classics such as "Casablanca" and "Citizen Kane" will be shown on a 20-by-40-foot outdoor screen, approximately drive-in movie size. The program, called "Screen on the Green," is an outgrowth of a popular event in New York and is being sponsored by Home Box Office and Banana Republic.

"Seven years ago HBO began the fun tradition of showing some of the greatest timeless films in Bryant Park. This year we are proud to bring this outdoor film festival to Washington where families can come and watch their favorite classics on a big screen with the Washington Monument as a backdrop," said Jeff Bewkes, the cable channel's chairman and CEO.

The National Park Service, which has to approve activities on monument grounds, has endorsed the festival. Local NPS superintendent Arnold Goldstein said the idea was "appropriate family entertainment" that would draw people downtown after work hours.

Watching films outdoors is a rare summertime event in Washington, perhaps because of the humidity and sudden rains. Nonetheless, the National Symphony Orchestra has successfully tried a similar format--a series of movie and music programs at Wolf Trap, which this year begins July 10. There, the NSO sets up two screens, slightly smaller than the ones planned for the Mall, and shows silent films, cartoons and scientific films with accompanying live orchestral music.

Michelle Boas, a spokeswoman for HBO, watches the weekly ritual from her office on 42nd Street in New York. "There's a whole culture to it. People bring wine and cheese, Scrabble games, and balloons to mark their place," said Boas. The cable company and Banana Republic also sponsored a festival in Boston earlier this week with "Citizen Kane" and attracted 14,000 people to Boston Common, she said.

The Washington screenings start July 12 with "Casablanca," that classic of romance and wartime intrigue. The screenings will be at sunset on successive Mondays with "The Wizard of Oz" on July 19, "Citizen Kane" on July 26, "King Kong" on Aug. 2 and "Rebel Without a Cause" Aug. 9.