IT SHOULD NOT GO unnoticed -- and won't, since nearly 500,000 people had good reason to notice -- that Washington continues to handle massive celebrations, demonstrations and gargantuan gatherings with increasing good frace and efficiency. The last big test, of course, was earlier this month, when America's birthday celebrants converged by the hundreds of thousands for the Beach Boys, fireworks and rites of the Fourth Downtown. Once again, logistically as well as pyrotechnically, that show was a smash.

Just how the Reagan and Barry administrations got the clouds cleared and the rain cut off on cue has not been made public. But a round of thunderous applause is clearly due the Park Service for its continuing efforts to see that amenities abound -- including a solid corps of courteous rangers and police officers to assist all participants at these jolly green lawn parties.

Underneath it all, and responsible in large part for the peopling as well as the clearing of the Mall, is Metro's subway system -- which, with the exception of a very few stalled trains -- rolled well. Even those who either didn't heed or didnht hear advisories against bringing cars into the area were greeted by helpful police officers who directed traffic to makeshift parking areas. Noteworthy, too, is the way the next-day cleanups are run; crews move in promptly to rake, plow and pluck clean every inch of the Monument grounds.

Probably the last thing that the cleanup crews want at this point is to be showered by a half-million thank-you notes. But they and the others around town who contribute to the festivity of these occassions should know that their efforts are appreciated.