David Del Tredici had made plans to come to Washington this week. He wanted to be on hand to hear Aaron Copland conduct the National Symphony Orchestra in his "Lobster Quadrille," which is one of more than a half-dozen works Del Tredici has composed on various aspects of "Alice in Wonderland."

What Del Tredici did not know until two days ago was that he and not Copland would be conducting "Lobster Quadrille." It happened like this:

Copland has been conducting down under in Australia lately. He also has a heavy conducting schedule in this country, with programs like the one he is doing four times this week the National Symphony. On that program, in addition to Del Tredici, he has programmed Samuel Barber's Capricorn Concerto, Irving Fine's Serious Song, and his own Third Symphony.

Copland has conducted "Lobster Quadrille" longer than anyone else, since he was the conductor at its world premiere in London on Nov. 14, 1969, which just happened to be Copland's 69th birthday. "Quadrille" is also dedicated to Copland.

However, as Copland looked over the Del Tredici score not so long ago, he remembered how very complicated it gets in spots. It is in seven parts, including two dances. As Del Tredici has pointed out, "The Quadrille was a kind of square dance in five figures and was one of the most difficult of the ballroom dances fashionable at the time Carroll wrote his tale."

In order to suggest some of the dancing of the Mock Turtle and the Gryphon, Del Tredici wrote a first dance, then a second, but wrote them in such a way, as he notes, that they "would make musical sense not only separately, but also when superimposed, one upon the other."

Dance No. 1 is played by brasses and strings; Dance No. 2 is for woodwinds and percussion. In between, there is a song for what the composer calls a "folk orchestra: banjo, accordion, two saxophones and mandolin."

Copland, faced with rehearsing the orchestra in two unfamiliar works, the Barber and Fine pieces, and his own demanding symphony, suddenly wondered out loud when he got to Washington, "Do you suppose Del Tredici would like to conduct his own piece?"

So a phone call was made Monday morning, and Del Tredici, who does not make a practice of conducting, but who certainly knows how to do it, said he would be delighted to be on hand later that day to see how it might turn out.

After rethearsals on Monday and Tuesday, everyone decided it was a great idea, which is how and why David Del Tredici, composer, became DDT, composer-conductor, a role he will fill for the rest of the week's concerts - today, tomorrow and Friday.