The Introit Gaudeamus

It doesn't take too long to find the answer to even the most obscure question any more! I went Googling (and a Hey, Nonny, Nonny...) and found exactly what I was looking for when I asked Ben about Gaudeamus used an an Introit. It comes from a chapter called "Josquin's Mass for All Saints and the Book of Revelation" in a book titled Symbolic scores: Studies in the music of the Renaissance:

It should be pointed out, however, that the Introit "Gaudemus" - as ascertained already by Helmuth Ostoff - is also used for a large number of saints' days as well as for the feast of All Saints (November 1). The Introit sung in the Mass of Saint Agatha (February 5) is the oldest version. Its text is the same as that of Example 1, except that it has "Agathae martyres: de cujus passione" (Agatha martyr, at whose passion) instead of "Mariae virginis: de cujus assumptionae" (the Virgin Mary, for whose Assumption). From the eleventh century onwards, the antiphon appears in at least seven other Masses. In the Introit of All Saints the text passage quoted above reads "Sanctorum omnium de quorum solemnitate" (of all the Saints, at whose solemnity).

Below is the "Example 1" referred to above:

[Another Edit: Ben to the rescue in the comments again! Still another of my questions answered, and I'll simply quote him directly again:

Regarding the Assumption: 'Gaudeamus' was the introit until 1950. That's when Pope Pius XII defined the dogma of the Assumption, and at that stage he replaced the ancient proper with a confected set of texts that more closely related to the dogma as defined, including the new introit 'Signum Magnum'. Not sure where the music for that came from. The wisdom of replacing wholesale an ancient festal proper is questionable, so in the revised Graduale Romanum in the 1970s, both Gaudeamus and Signum Magnum are offered as alternatives.

Thanks a million, Ben!]