Do you BUY domains at a certain time of day?
From what I've seen, and I'm no expert, they tend to drop whevenever... no particular time or day seems best.
For most, they just use a drop service like SnapNames that grabs them up whenever they drop... so you don't have to sit and wait. Of course, with all the drop services out there, the chance of you hand reging a domain at drop is slim to none. They will get reg's for 3 - 5 days just to see their traffic and THEN dropped by the snapper if they don't perform.
So the short of it is search any time... most companies will have lists 3 - 5 days in advance publically available.
GoPC
