Verdi composed his Messa di Requiem in 1874, in the midst of a pause in operatic composition (he premiered no new operas between Aïdain 1871 and Otello in 1877).
Verdi composed his Requiem in honor of Italian poet Alessandro Manzoni, who died in Milan in 1873.
The Libera me of Verdi’s Requiem dates back to around 1878, when Verdi suggested that “the most distinguished Italian composers” collaborate on a Requiem for Gioachino Rossini. The project was never performed, and Verdi ended up composing a whole Requiem on his own instead, integrating the Libera me which he had already written.1
