WHAT WITH SUBWAY construction and the great retooling of the Grand Avenue downtown, various parts of Washington have been hiding behind boards or altogether missing for years now. But every once in a while an old block or two comes bounding back into view, looking greatly improved and ready for full-fledged public use. So it was over the weekend, when the big centerpiece of the Pennsylvania Avenue redevelopment plan -- the mighty square between 13th and 14th Streets NW -- came back to active life under its old name of Western Plaza. With it came the ingredient that makes an ambitious design acceptable: people.

True, there was special bait to draw a crowd -- the opening featured music, dance, theater, roving performers and the sweet smell of successful cooking from 12 city restaurants and, as icing, a 65-pound cake. But the new plaza, which includes a reproduction of Pierre L'Enfant's plan of the city, does turn out to be far more inviting than the cold concrete "Red Square" that was once planned to dominate the Pennsylvania Avenue Plan. As part of a design with a mixture of old and new architecture, Western Plaza should add to the allure of this area for tourists and townspeople, new businesses and the old Post Office Building.

The redevelopment of the avenue is now one-third complete, and by Inauguration Day two more parks will be completed. Work along the remaining parts of Pennsylvania Avenue is set for early 1981 and, if all goes well, there will be life in hotels, shops, offices, apartments and maybe even the Willard Hotel. Gone, we hope, will be the creepy-quiet, lifeless nighttime atmosphere along this main street. Though there are bound to be parts of this plan that miss the mark or fail to win unanimous praise, Western Plaza could be the start of something big -- and good -- for Washington.