From the New Advent Catholic Encyclopedia

(From novem, nine.)

A nine days' private or public devotion in the Catholic Church to obtain special graces. The octave has more of the festal character; to the novena belongs that of hopeful mourning, of yearning, of prayer. "The number nine in Holy Writ is indicative of suffering and grief" (St. Jerome, in Ezech., vii, 24; -- P.L., XXV, 238, cf. XXV, 1473). The novena is permitted and even recommended by ecclesiastical authority, but still has no proper and fully set place in the liturgy of the Church. It has, however, more and more been prized and utilized by the faithful. Four kinds of novenas can be distinguished: novenas of mourning, of preparation, of prayer, and the indulgenced novenas, though this distinction is not exclusive.