Monday the 13th day is a modern worker's version of Friday the 13th. Mondays are considered hard and depressive days (after the fun weekend it's often not at all nice to return to work); Logically, then, no one (of the superstitious people, at least) can expect anything but troubles in work in Monday the 13th.
(Naturally, dissatisfaction isn't guaranteed. This is just psychology.)
Popularized by Garfield, who hates Mondays. (Don't we all? =)
See especially: Monday, thirteen