"Alleluia" occurs only once in the New Testament, and that is in the Book of Revelation (19:1-9).
In the early Church, the "alleluia" was sung every Sunday, but by the fifth century, it was used only at Easter. Eventually its use extended to all liturgies, except during Lent. It was then replaced by a psalm chant known as the tract (a solo chant sung all the way through without any repetition).
- Little White Book, Diocese of Saginaw