Sod’s law:
„the principle that what can go wrong, will go wrong, usually with some observed degree of irony
Sod’s law that the burgers are burned on the outside but raw in the middle“
http://en.wiktionary.org/wiki/Sod%27s_law
or Murphy’s law:
„An adage which states that if anything can go wrong, then it will.“
http://en.wiktionary.org/wiki/Murphy%27s_law
In my personal experience both of them appear more frequently on Mondays.