When is the best time to practise gratitude?

Both morning and evening work, for different reasons: morning sets a tone before the day crowds in, and evening lets you end by staying with something that went well, which can also settle the mind for sleep. More useful than the slot is the condition you are in when you arrive. This practice needs something with a bit of charge still on it, so soon after the thing happened tends to work better than a scheduled review much later. If you find you are hunting for material rather than returning to it, the timing is probably wrong rather than the practice.