It's the evening of December 17. It's time for the O Antiphons. These are the seven antiphons that in Catholic traditional usage are chanted preceding the Magnificat during Vespers on the seven days preceding Christmas Eve. These each address the Messiah in a different title drawn from the Old Testament, calling upon him to come to Israel:

O Sapientia — O Wisdom
O Adonai — O Lord
O Radix Jesse — O Root of Jesse
O Clavis David — O Key of David
O Oriens — O Dawn
O Rex Gentium — O King of the Nations
O Emmanuel — O God-with-us

Each of these is the basis for a verse of O Come, O Come Emmanuel (though as usually given in English there are only five verses), but Emmanuel, the first verse of the hymn, is the last of the antiphons, making the hymn most appropriate to sing on Christmas Eve before nightfall, or on the preceding evening after Vespers, as then one can look back on the whole series of antiphons and, looking only at their initials, see that in retrospect they spell ERO CRAS, which in Latin means "I'll be there tomorrow", the promise of Advent's close.

A good source for the full texts of the antiphons and further information on their history and use is the Kir-Shalom O Antiphons page.

Liland