Friday night and Sunday especially, as that's when the weekend warriors are about. About 2am UK time used to be best for me, as there were more South Americans online by then.
Part of the reason I barely play on Stars any more is the "Eastern European nitbox" phenomenon. (Not that I have a personal problem with people trying to pay their heating bills. I just don't like what's happened to online poker.)
When it's 1pm on a Monday in Belarus, you may as calculate how much rake you'd pay per hour, and transfer it directly to David Baasov instead of playing.
I mostly play on 888, and very occasionally on iPoker when they offer me some free money or tourney tickets, but I'm probably gonna try another network soon. Since I'm primarily a recreational player, and not a mass-tabler, there are lots of options. Unibet seems like a decent option, but there are plenty of other sites I need to investigate.
Came here due to Unibet mention obviously (hurrah!)
I think that a lot of people overlook the day of the month. Sure, a random Friday evening will be better than a random Tuesday afternoon, but it's more important to play shortly after payday. This is going to vary by site, but figure out where most people on your site are from and play the day they tend to get paid.
I also think that Saturday or Sunday afternoons aren't that different from weekday afternoons.
Both of these ideas are backed up by PokerScout numbers and site data by the way, so I'm not sure why they aren't more widely spoken about.
Friday night and saturday night after 8pm UK time.
License to print money with all the drunk Russians on at this time and later on all the drunk English.
Well it is really hard for me not to be drunk at weekend lol but when I'm not drunk it is really profitable to play heads-ups. I don't know why but heads-ups are much softer than ring games for me at sat/sun evening.