This anonymous chant dating to the seventh century has traditionally been sung during Advent, specifically during Vespers.
the stars of night,
save us all,
and hear Thy servants when they call.
Thou, grieving that the ancient curse
should doom to death a universe,
hast found the medicine, full of grace,
to save and heal a ruined race.
It provides for a sober prelude to the liturgical season of the Incarnation with the full breadth of salvation history -- the beginning, the middle, and the end.
From the sin of Adam ...
Thou camest, the Bridegroom of the Bride,
as drew the world to evening tide,
proceeding from a virgin shrine,
the spotless Victim all divine.
... to that which at the heart of said history, and so is at the heart of these words, the coming of a Savior Who would loosen the bonds of death to sin ...
At whose dread Name, majestic now,
all knees must bend, all hearts must bow;
and things celestial Thee shall own,
and things terrestrial Lord alone.
... and on to the certainty of judgment at the end of all time, as told in the Gospel account for this first Sunday of the Advent season.
O Thou whose coming is with dread,
to judge and doom the quick and dead,
preserve us, while we dwell below,
from every insult of the foe.
To God the Father, God the Son,
and God the Spirit, Three in One,
laud, honor, might, and glory be
from age to age eternally. Amen.
In 1632, Pope Urban VIII's commissioned a revision of many of the hymns of the Roman Breviary. Those for Advent were no exception, and were significantly altered, so as to better conform to classical Latin poetry and meters. Only the first two lines shown here of the original remained. The revised hymn, titled Creator alme siderum, is a separate hymn in and of itself.