The best time to do cardio is the time when you feel most energetic throughout the day,this allows one to use all the energy and to maximise the effectiveness of the workout.To get the best results for cardio training,it is ideal to separate your cardio training days with weight lifting days,this is best for guys who need to add weight but for those who's priorities is not to gain mass ,it is alright to do cardio before doing a low intensity weight training session.Most people do it wrong by combining cardio and weight training,if you do your cardio training before lifting weight can deplete your glycogen stores before and defeat the purpose of your weight lifting session.If one is doing cardio and resistance workout done in as a single session,you are better off doing your cardio session after your weight training.

Glycogen is carbohydrate produced by the body and stored in the liver and muscles as energy reserves.Mostly it is stored in the muscles so it can quickly use it for energy and if you have few of them you will not be able to push yourself. 

Avoid doing cardio on an empty stomach,our body requires energy during cardio session and whenever the body lacks energy due to no source off energy "Food" it will start to look elsewhere for energy.It actually gets energy from your muscles.It is better if you have a snack 30 minutes prior to your cardio training.