WHEN OPPORTUNITY knocks, it's more often on the doors of comfortable homes -- where the resources exist to open still other doors to valuable as well as pleasant experiences. Those Washingtonians who have been fortunate enough to travel abroad -- and to send their children on similar memorable trips -- know how rewarding such times can be. But they should know, too, that with their help, some dedicated young people of this city could share in what would be a once-in-a-lifetime adventure -- while bringing great credit to their country in general and the capital in particular. We're talking about the members of the award-winning Eastern High School choir, which happens to be -- and deserves to be -- invited to represent the United States in the International Youth and Music Festival in Austria next July.

To go, they need help. And for help, they have begun serious fund-raising activities. To sing for their international suppers, the more than 60 voices of the choir, under the direction of Joyce Garrett, are conducting a city fair/carnival at RFK Stadium, through Sept. 19, with rides and other activities for children and enough to interest adults as well. They're also accepting contributions from those who cannot attend but who wish to help. The Eastern Fund-Raising Committee, headed by Maudine R. Cooper, is handling these donations. For further information, donors may call Vice Chairman Tom Howell at 246-0271 (office) or 567-0629 (home). With enough help, the group will fly.