People often have big nights out on fridays and it causes a night of bad sleep. Run-down bodies are more prone to illness and thus sickrates are higher on weekends.
For those with kids who do not go out friday nights, the children sleep later than usual and thus so do the parents meaning they do not get enough sleep and are tired over the weekend. Having to entertain the children all day adds to this and then they are more prone to illness and thus sick rates go up.
