A rare opportunity for any concertgoer is coming to National Symphony Orchestra subscribers this week. In order to enjoy it, he watchword is, "Don't be late!"

James DePreist, formerly the NSO's principal guest conductor and now music director of the Quebec Symphony (which he recently led in an impressive program in the Kennedy Center), has announnounced an unusual opener for this week's subscription concerts. It is the Funeral Music from the "incidental" pieces Berlioz wrote for "Hamlet."

Shakespeare was, among all writers - and even more than the Virgil whom Berlioz adored - the mightiest literary influence on Berlioz. The composer once wrote to his friend Count Michael Wielhorsky, "The English are quite right when they say that Shakespeare is the supreme creator, after the Almighty."

When Berlioz first saw "Hamlet" is Paris, during a visit by the Abbot Company in 1827, his life was changed forever. For on that night, as he later noted, he saw for the first time the woman he would eventually marry. That it look five years for the marriage to occur is, thanks to hindsight, a clue to the unhappiness that soon set in. But Berlioz could know nothing of this when he wrote in his memoirs, "I was at the first night of 'Hamlet.' In the role of Ophelia I saw Henrietee Smithson, who five years later became my wife. The impression made on my heart and mind by her extraordinary talent, nay her dramatic genius, was equalled only by the havoc wrought in me by the poet she so nobly interpreted. That is all I can say."

Others said even more of the Irish-born Smithson as Ophelia. "Some spark of genius in her responded to the mood of the moment," one observer recalled. "Her acting in the mad scene, which so moved the house that men wept and left the theater convulsed by uncontrollable emotion, seems to have been almost an improvisation, owing nothing to traditional routines."

No wonder, then, that Berlioz, when he came to write music for the final scene of the drame, in which Shakespeare directs. "Let four captains bear Hamelt, like a soldier, to the stage," created en epic moment. To an orchestra muffled in somber hues, Berlioz adds a chorus whose sorrow is expressed in, wordless sounds. The scene is not long. It does not need to be.

But within its confines, Berlioz achieves a mood that parallels that in the Funeral Music Wagner wrote for the murdered Siegfried. Unlike Wagner's muisc, however, the Berlioz scene is rarely performed, either in concert or even on recordings. This is not due to any question of quality or grandeur, but rather, undoubetedly, because of the requirement in its brief span for an expert chorus, sensitive to the unusual demands of the music.

Thus DePreist is performing a distinct service in bringing this music to National Symphony audiences this week. He will follow it with the Bartok Concerto for Orchestra, and, with Claudio Arrau as soloist, the Brahms D Minor Piano Concerto. Remember. The Berlioz music will be finished within 10 minutes of starting time. Why be late?