I'd gather it's based on the 18th chapter of the Book of Revelation, especially the passage that begins with verse 21. Babylon was looked upon as the ultimate oppressor, since it destroyed Jerusalem in 597 BC, and kept the Jews captive for almost 60 years. The "day so long expected" is probably the "Day of the Lord," "Armageddon," "Judgment Day," or whatever you want to call it - "the very last day." Your standard eschatological theme, Alison. You've got the idea about right.
-Joe Offer-