Bitcoins is when the market price is low as compared to normal price. As far as selling is concerned, I think there is no better time for selling but the time when you are getting enough amount of profit by giving away Bitcoins. Any investment is influenced by demand and supply. But the major ifluencer are beliefs. Do the masses believe that prices go up or down. Believes are a complex construct and can be changed instantly, for instance due by the spread of good/bad news