Popular myth says exercise is best done in the morning. But the truth is there is no specific "best time of the day" for exercise.
So if you're not a morning person, don't even think about exercising in the morning. The same thing goes for people who don't have time in the afternoon or who are completely exhausted in the evening. Exercise when you can, not when you think you should. There is no "best time" and no specific time when your body will burn fat more effectively.
Another related myth is that you shouldn't exercise just after you've eaten. It is true that exercise after a large meal may interfere with digestion as energy is directed away from the digestion process and towards the exercise; this can result in a slight stomach ache. However, a comfortably-paced walk after a meal won't do you any harm, and may help to relieve some of that bloated feeling and restore some energy.
Please note: If you do experience extreme tiredness or discomfort while exercising after eating after a meal, notify your doctor as it may be a symptom of undiagnosed hypertension.