NEW YORK (AP) _ All during 1991, music organizations have been performing Mozart, marking the 200th anniversary of the death of the great composer. Many scheduled performances for Thursday, since Mozart died on Dec. 5, 1791, a few minutes after midnight.

The Richmond Symphony played Mozart's ''Horn Concerto No. 4'' with soloist Barry Tuckwell at Kennedy Center on Wednesday evening, figuring that was closer to Mozart's time of death than a Thursday evening concert.

The New York Philharmonic played Mozart's ''Requiem,'' magnificently, in a soul-satisfying concert conducted by Erich Leinsdorf on Thursday.

The concert in Avery Fisher Hall at Lincoln Center began with Mozart's brief ''Masonic Funeral March'' from 1785. Alicia de Larrocha was soloist for ''Concerto for Piano and Orchestra, B-flat major,'' Mozart's last piano concerto, dated Jan. 5, 1791. It is a life-affirming, optimistic-sounding work, heard between the funeral march and the requiem. De Larrocha gave the piano a light touch, making the music ripple like a brook through woods, not emphatic but silver and quietly proud.

The ''Requiem'' is affirmative also, not sad or ponderous. It was Mozart's last composition, which he struggled unsuccessfully to finish as his health and strength were failing.

Leinsdorf, who last conducted it in 1964 at a memorial Mass for John F. Kennedy in Boston, studied the various completions and editings. Leinsdorf says in a program note that he took what he deemed best ''solely according to my taste - which has only an endorsement of 60-odd years playing, coaching, conducting and recording Mozart.''

Soloists were soprano Roberta Alexander, who has a beautiful tone, mezzo- soprano Florence Quivar, tenor John Aler and bass Paul Plishka. Aler has the closely focused tone which says ''Mozart specialist'' though he sings a broader repertory than that. They blended exceptionally well when singing as a quartet.

The 40-voice New York Choral Artists was the choir.

Lincoln Center, which embarked last January on performing all of Mozart's music as a bicentennial tribute, couldn't get it all done in one year and will continue through the first six months of next year.