First thing in the morning. It has to be this way or it won't get done... after work/school is too hectic and I want the evening with my kids at home. I basically get up at 5 a.m. or sooner. If I am going for a run, I'm out by 5:20 and back by 6:10 during the school year. If I'm going to the gym, I try to get there by 5:15 and back home by 6:10. Luckily, the gym is only a 4 minute drive from my house. My boot camp teacher said that I don't have to eat anything before the workouts because they are so early, but if you are not going to work out till a half hour or more after waking, you should eat a little something prior to the workout. If you are not working out upon waking, be sure to eat within a half hour/hour of waking up so you don't slow your metabolism and systems.
I hate waking up so early during the school year sometimes but I am thrilled when it's noontime and I realize I am done with the workout for the day. Find what works for you... good luck!