| Mozart composed the motet Exsultate, jubilate for Venenziano Rauzzini (1746-1810, also called Venanzio?), the lead in his new opera Lucio Silla, while they were in Milan performing it. It was premièred in the church of San Antonio on 17 January 1773. (Mozart was 16.) Its opus number is K.165.
Rauzzini was a castrato so it is now sung by sopranos of course, or mezzos.
It is an immensely sprightly piece, for strings, oboes, and horns. It dances along and drops into winsome rhythms with the lightest of touches. It makes me think of Salieri listening to it and despairing that anything so simple could be so far beyond what he himself could write.
The text of the motet is Latin, of unknown origin; it was known to be in the court at Munich in 1753, and it might have been Rauzzini who gave the text to Mozart. The motet was written for the Nativity. In 1978 two different versions were discovered in a hamlet near Salzburg, one for Trinity and another for Nativity.
Exsultate, jubilate,
o vos animae beatae,
exsultate, jubilate,
dulcia cantica canendo;
cantui vestro respondendo
psallant aethera cum me.
Fulget amica dies,
jam fugere et nubila et procellae;
exortus est justis inexspectata quies.
Undique obscura regnabat nox,
surgite tandem laeti qui timuistis adhuc,
et jucundi aurorae fortunatae
frondes dextera plena et lilia date.
Tu virginum corona,
tu nobis pacem dona,
tu consolare affectus,
unde suspirat cor.
Alleluja
Exult, celebrate,
O you blessed souls,
exult, celebrate,
singing sweet songs;
in answering your singing,
the heavens sing out with me.
Friendly day is shining,
now fled are clouds and storms,
for the just an unexpected quiet has arisen.
Everywhere dark night was reigning,
come forth happy you who were afraid before,
and joyous for the fortunate dawn
give lilies and garlands with open hand.
You crown of virgins,
you give us peace,
you console our feelings,
whence our heart sighs.
A detailed analysis of the alternative versions can be found at http://pzweifel.com/music/exsultate.htm | 
