wordplay

I grew up in a Protestant denomination and at one time had most of the New Testament memorized and this is a question I have no idea about the answer for. Is there a reason that God sent Jesus when he did? Is there a timeline? Some kind of triggering event? Some "fullness of time" thing? IS there a standard story? M. (who was raised Catholic) said that he thought it had something to do with Purgatory filling up or something?

I got one Google hit that had a story that basically said that he picked that time because he knew that he'd get the best crucifixion -> salvation story out of it. Uh, is there a more satisfactory explanation out there for this? *tries to be respectful*