Dental floss is one of the best oral hygiene maintaining techniques, which is sometimes said to be better than tooth brushing, only due to the reason that the regions which can not be reached by brush, can be accessed by the floss thread, and it effectively removes the bacterial dental plaque from those regions, and minimize the risk of dental caries.
So, coming to the point of when to floss –
It is the night time before bed, when you can get the best use and results for flossing. Because it is the time when saliva in the oral cavity is less in amount, and the conditions are favorable for bacteria to grow and form dental plaque. The prevention of dental plaque or removal of freshly formed plaque can keep your teeth bacteria-free till next morning when you brush your teeth.
It is advised to floss at night time, though if you find time, you can do it after brushing in the morning too. But the most dental plaque formation is during night time as there is no activity in the mouth and bacteria can grow indefinitely at that time.