sometimes on a tuesday night after midnight is good. cause for some reason when the wars start and the rest of the airlines jump in by wed lower fares will be put out there! and the last weekend of the month usair details a flight to see how the aircraft is filled and then that also opens up seats and right after a schedule change as well. because fi they switch aircraft around and move a bigger aircraft to where they had originally had a smaller one flying then there are more seats as well. there is a book called the travel detective by peter greenberg thats pretty good. not all of his info is accurate but he is pretty good. this is all he does for aliving travel and tests the waters to see what is out there and how to get this and that etc.
personally I htink anytime between 1-4 am is the best time cause this is when they load everytying into the computers at the airlines.....