The Gregorian introitGaudeamus omnes is among the oldest melodies of the repertoire, and with minor adjustments is used for several different feasts in the Latin rite: the California missionary Narciso Duran went so far as to adapt it to all 52 Sundays in a choirbook preserved at Berkeley's Bancroft Library.
Let us all rejoice in the Lord celebrating the feast
Variant 1 (Assumption)
in honour of the blessed Virgin Mary
in whose assumption the angels rejoice,
while the Archangels praise the Son of God.
Alleluia.
Let us all rejoice in the Lord,
celebrating the feast in honour of all the saints
in which the angels rejoice,
while the Archangels praise the Son of God.
Ring out your joy to the lord, O you just;
for praise is fitting for loyal hearts. Glory be to the Father ...
Let us all rejoice in the Lord,
celebrating the feast of Saint Stephen
in which the angels rejoice.
Let us all rejoice in the Lord,
celebrating the feast of Saint Thomas the martyr
in which the angels rejoice, praising the Son of God.