The best time of day to play CS:S is around 00.00 EEST (21.00 GMT/UTC).

The theory is that before the midnight there are too many children playing. They don’t grasp the larger picture of the game. On the other side, the early hours, there are too many players from weird countries who probably don’t even know what game they are playing. Then there are players who are too sleepy or something to do anything wise.

The skilltime graph gives certain view over the players who are playing at certain times. It is probably connected to the perception I’ve come to.