As I understand it....
Cortisol helps break down bodily tissue, whereas Testosterone helps build it.
Testosterone levels are normally highest in the morning (insert witticism here).
However the ratio of testosterone:cortisol is highest:lowest in the evening.
So in the evening time your body may be in a more naturally anabolic state, meaning it is more predisposed to build muscle.
I train at all different times of the day and normally find early morning or late afternoon to be most productive. My best lifts have usually occurred in the afternoon. You may be different though, so experiment with what works.