Saturdays are usually great for upgrades. Most times all medallions will clear and even have a few NRSA's clear. For me, Sundays have also been pretty good and then followed by Tuesday.

I also try to stay off the beaten path. Avoid ATL as much as a possible. I try and connect through CVG and JFK as much as possible. They have been excellent for upgrades. I only missed one upgrade this year out of CVG and I have never missed an upgrade out of JFK.