According to the Book of Common Worship, the "O Antiphons" are liturgical prayers for part of the season of Advent. They are seven prayers that each start with "O" and some name for Christ. "O Wisdom," "O Lord," "O Root of Jesse," etc. The gimmick is that when you list them in reverse order and in Latin(!), it spells the Latin phrase, "ERO CRAS," which means "I will come [tomorrow]." Think of it as an eighth century, geeky, monastic form of an Advent Calendar to help you count down the days until Christmas Eve.
We'll be praying the O Antiphons as part of the Morning Prayer Service at 8:00am in the sanctuary. Normally, we only have the Morning Prayer Service on weekdays, but the O Antiphons are designed to be prayed from the 17th to the 23rd of December. If we skipped Saturday and Sunday, we'd only have E-- CRAS. I don't know Latin well enough to translate that, but it's not what's intended.
So we'll have a special Morning Prayer Service on Saturday, December 18th and Sunday December 19th in addition to the normal ones on weekdays. If you've been interested in the Morning Prayer Service and haven't made it at 8:00am yet, you can use the O Antiphons as a special event to motivate you to try it out.
I look forward to seeing you there as we use this special litany to help us prepare during some of the last days of Advent.