I believe the best time to plan is in the night before the day break.
One of the most important factor of kick starting a productive day is knowing what your priorities are the moment you open your eyes.
By having a outline of what you want to accomplish, you can get straight to work
Without this, you'll likely having a very relaxed start to the day while you are try and get around to actually deciding what you want to do. Losing your Potential to start the day running
Leaving a notepad near your bed, and build a habit of writing down small list of priorities to tackle when you wake up the next day
Watch how much easier it will be to become laser focused on your goals, when you know they're waiting for you the moment you wake up
yeah, I think night is best choice, if you already know what is waiting you tommorow.