What is the worst day to make significant changes to your network? For users it is probably Monday, but for any system administrator the worst day to make network changes is probably on Friday. Invariably things go wrong when you make changes. So from an administrator's point of view, staying late during a weekday is infinitely preferable to spending your weekend trying to fix problems. At best you are available and can make the fix, at worst you are off leaf peeping for the weekend and unavailable as the problem escalates.
It seems reasonable to make changes on Friday, because Fridays can be the lightest networking day of the week. However, by late Friday afternoon people and resources that you need internally start to become unavailable. Externally, you will find that not only are parts increasingly difficult to find, but many of your service contracts may specify either no weekend service, service at greatly increased rates, or that the service provider sends someone from the "B Team."
When a problem arises that is the direct result of a change, you will also find that without the full complement of clients working on the system bugs don't get noticed and fixed as quickly as well.
It's worth talking your management into the idea to never make changes on a Friday.
Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.