When is the best time of day to exercise?
The simple answer is whenever is best for you.
Whether you like to get up early and get your workout done before the school run, or you have access to a gym at lunchtime, or you prefer to unwind with a workout after work, whatever your preference is, stick to that. You'll be much more likely to make exercise regular occurrence in your weekly routine if you do it when best suits you, and as you've heard me say many times, consistency is key.
Don't forget, if you find you haven't got time for a dedicated training session one day, you can keep the momentum going by moving as much as possible outside of the gym environment, and one missed gym session won't be detrimental to your results. Just don't let one missed session turn into two, or three etc. No good can come from that.