Use of The Incipit
In the same way as Church documents are referred to by their incipit (their first words in Latin), Mass formulas are known by the incipit of their Introit, which is the first text in the formula. Thus a Mass for the dead is referred to a Requiem Mass, and the three Christmas Day Masses have been called Dominus dixit, Lux fulgebit and Puer natus. So too, Gaudete Sunday is a name for the third Sunday in Advent, Laetare Sunday for the fourth Sunday in Lent, and Quasimodo Sunday for the Octave or Second Sunday of Easter, because of the incipit of the Entrance antiphons of those Sundays.
Read more about this topic: Introit