Why do people have good and bad days?

It is because of their mood or personality they're feeling on that particular day. If they slept late, or couldn't sleep, this could cause them to have a bad mood, and because of their attitude, they'll have a bad day. Also, sometimes if you're feeling bad about yourself and think you're unlucky or think negative, then you'll actually have a bad day, vise versa, if you have a positive attitude on that day and feel confident, then you'll most likely have a good day.