Fire on Mondays. This lets dismissed workers start looking for a job right away.
Make job offers on Thursdays. If candidates need time to think, you can give them one extra day. If you give them the whole weekend, they may find another offer.
Give good job reviews on Fridays. It sets the mood for a good weekend, which can be a reward in itself. It also prevents satisfied workers from "kicking back" for the rest of the week.
Give poor job reviews on Mondays. This provides employees time to work out improvements during the week, instead of stewing about them all weekend.