The “O Antiphons”, a collection of antiphons sung with the Magnificat at Vespers from 17 to 23 December in the Roman Rite, date back at least to the reign of Charlemagne, and there is an English poem based on them by Cynewulf (fl. 8th century). The well-known Advent hymn, “O come, O come, Emmanuel” (Veni, veni, Emanuel), is a metrical text based on the antiphons.
The initial letters of the antiphons,
ignoring the ‘O’, spell out the reverse acrostic, ‘SARCORE’ – ‘ero cras’, ‘I shall be [with you] tomorrow’, a hidden counterpart of the joyful iteration of ‘cras’ which rings like a bell through the liturgy of the last week of Advent.
(The New Oxford Book of Carols, ed. Hugh Keyte and Andrew Parrott, p. 45)
For December 17
quae ex ore Altissimi prodiisti,
attingens a fine usque ad finem fortiter,
suaviterque disponens omnia:
veni ad docendum nos viam prudentiae.
who proceeds from the mouth of the Most High,
reaching out mightily from end to end,
and sweetly arranging all things:
come to teach us the way of prudence.
(English translations of the Latin taken from the Dominican Life website.)