This happens in many different niches/topics, it's not abnormal. Many people are away from their computers, spending time with family, busy doing things they couldn't do during the work week, etc., so this happens often and it can even be regular.
Check this thread: Do online sales drop on weekends? Need your experience I imagine it depends on your demographic and what your site is about. How do the numbers compare with this time last year?
it's my first month ass affiliate marketer, I did not have enought data to analyze this period of time but in hope next weekends will be better, I think number of sales mostly depends on luck , lets say 60 percent experience and 40 percent luck