Seven years of misfortune is a common superstition that dates back to over 2000 years in the ancient Rome and Greek. After they learned to manufacture mirrors it came a common place to believe breaking one would cause seven years of misfortune.
The misfortune itself comes from the belief that mirrors were gateways through which gods could observe the mortal world. So it was considered highly disrespectful to break one of gods windows to the world.
But why seven years of misfortune? Well, those ancient people believed the body renews itself every seven years thus ending the curse of misfortune when the person becomes completely new.
The fact that people believed we renew our bodies every seven years already two thousand years ago is quite a coincidence. In the 50's a study conducted at Stanford university found out that all the cells in our body regenerate within the span of seven to ten years.
That means given I'm 43 years old I'm currently living on my seventh body.