O Come, O Come Emmanuel, is a hymn that incorporates all of the previous seven days worth of chants, commonly referred to as the 'O Antiphons'.
When listed in reverse order, in Latin, the initial letters of each antiphon spells the Latin phrase Ero Cras.
Emmanuel
Rex Gentium
Oriens
Clavis David ERO CRAS translated means "Tomorrow I will come"
Radix Jesse
Adonai
Sapientia