Best Email Time

Ok, so the question of when is the best time to "spam" has come up. I cited the ReturnPath 2004 study (http://returnpath.biz/pdf/time_deliverability_0704.pdf), but now the question of when we think the Net is most congested (more likely to see overloaded MX servers and delivery failures?). Anyone have any data on such? Sorry if this question seems offtopic here, but I figure the question of net congestion data is appropriate. -Dennis