Are there certain days in a month that you're really blue?

Yeah, I think that there are some certain days when I don't feel so high in spirit. I still remember when I was at college learning the subject psychology, my teacher told us about the curves of each of us individuals having some special days when we feel letdown or blue, and that it is quite natural for us to have this happen. But this is also a very important period when we are supposed to be more cafeful in our life and work as it is not easy for some people and usually something bad would easily take place when those who could control themselves well...