My understanding is that, in older times, a lustrum was the sacrafice offered after the Census, taken every five years. That is how it came to take it's current meaning. However, the Sybil, when she wrote her books, must have still kept to the ancient meaning of the word. I then propose that we move forward with the Census, offer a purification, and then the Censor offers the traditional sacrafice, the 'lustrum', which your prophecy speaks of. I think that would appease the gods, and thus is my suggestion for the course of action which this house should take.