If I hear that Mercury is in retrograde, I get really nervous. I actually believe that some astrology is real and can have an effect on people. I don't believe in the weekly horoscopes, but I do believe that the alignment of planets and stars mean something. Think about the relationship between the Moon and tides, for example.
When Mercury is in retrograde, the effects can last for a few days. I try not to do anything major during that time, like take a long trip or plan an event. There's just too many chances of something going very wrong during that time.