As suggested by the answers in this question on fitness.se, the best time of the day is whatever works best for you.
There is no magical time that will boost your metabolism more than other times, or anything like that. What most people are thinking of when they think of metabolism boosting is EPOC, or excess post-exercise oxygen consumption. This is commonly called the "afterburn" effect, which is where the body consumes more calories for an extended period after exercise sessions. The more intense the session, the higher the effect.
However, even with that, the amount of calories is low, in the 10-100 calories range over an extended period, as shown by the studies and math cited in this article.
The best thing to do (as suggested by the AHA) is to find a time that is most convenient and enjoyable for you, as you are far more likely to engage in the activity on a consistent basis, and being consistent is one of the best things you can do as far as exercise is concerned.