How I could know what is the best day to fish

Well a bad day of fishing is still better than a good fay at work. So anytime. But seriously though if you are looking to try and predict peak activity levels try the solunar table. This is often used to predict the peak feeding times of most species of fish. It also depends on the type of fish that you are fishing for. For example I like to fish muddy watter after a storm for catfish.