Well, considering that most of the player base appears to reside in the United States or Northern Europe, I'd say around 8:00 (20:00(?)) GMT on a friday, since that would be a reasonable time for UK/Ireland/France/etc. users to be awake, and would be the perfect time for US residents to be coming home from school (younger users, that is); or a few hours earlier than that on a satuday/sunday as there would be no school (or work depending on where you live), and more people would be available.