It's an old expression: " Fish and houseguests smell after three days". It's comparing houseguests to fish - after three days, they've been there long enough and they're over-staying their welcome. You get tired of smelling fish after three days and you want that smell gone; likewise, you want your visitors gone after three days.

I would like to add , that I don't agree much . Visitors don't necessarily have to be a burden after a certain number of days. It really depends on the visitor,some you would like to get rid of after an hour and some you might like to extend their period of visit and love them to stay longer.

This is a very old proverb coming from times when people's houses were cramped and means that no host can be hospitable enough, to prevent a friend who has descended on him, from becoming tiresome after three days. In this proverb,the visitors are compared to fish which stank after 3 days, because they were not able to be refrigerated back then. Read more: http://www.answers.com/topic/fish-and-guests-smell-after-three-days#ixzz1QQ4jzXI4