I'm not 100% sure what time but in the past it has been around midnight?
I know I will be on for the whole day though.
The first few games I will be playing solo then I will play with randoms or people on my friends list. If you are on my friends list don't get mad though if I don't join. I'm just trying to find other players to add on my friends list. DLC release is the best time to find them IMO.