When should I meditate? What's the best time to meditate?

"Life is a present which I unwrap every morning at waking." Christian Bobin 

"We think that we take our dog for a pee at noon and in the evening. Big mistake: it is the dog that invites us out twice a day for meditation." Daniel Pennac 

It's really up to you to decide when you can meditate. There's no good or bad time. You simply have to make it a habit, a routine that forms part of your everyday life. Some people meditate in the morning when they wake up. Others after their shower, or when they arrive at work. Others still after lunch, during lunch, after dinner, after a workout, in the evening when they get home, while walking the dog, before sleeping... In other words, there are as many possible 'best' times as there are meditators. Find what best suits you and stick to it. Choose a time when you aren't likely to be disturbed, when you can be comfortable and aren't too tired, when you need calmness and clarity... 
In the United States, meditation teachers often recommend the RPM method: Rise, Pee, Meditate. ;)

We also suggest that, if you like, you extend the experience with brief informal meditations (10 seconds to 1-2 minutes) at any point in the day to reconnect with yourself. For example, at the start of a meeting, as you walk along a corridor, on public transport, while eating...