We have fields/gyms reserved so everyone knows that they will have practice on certain days and it can be either 5:00, 6:00, or 7:00.

If they can't make those days/times work, then they shouldn't be signing up for the league.

Now, if you're giving latitude to each coach to practice when and where they want, that creates a whole separate problem and I guess the only solution is to tell people that if they want certain days/times, then they can volunteer to coach and pick the day/time they want.