The two big GDS systems are in Atlanta (Worldspan) and Dallas (Sabre). It depends on which airline you're looking at as to which system has the cheap fares. I've seen fares on Sabre that have already expired on Worldspan. I would think (correct me if I'm wrong) that fare periods are based on GMT (Greenwich Mean Time) which is what the rez systems run on. Expiration of "held" fares is based on local time of the agency. Which explains why Expedia expires res at 3am EST.
I concur on the Wednesday thing. Best thing to do though is check EVERY morning. Knowing the usual fare to your destination helps. And read the fare rules associated with a fare. They will give you some hint as to the logic behind how long a fare will be offered and how desperate the airline is to fill seats.

My recommendation is buy Wednesday/Thursday, avoid Saturday-Monday.