Breaking a mirror is seen as seven years of bad luck from a historical root.
answer : 
Back in the medieval times mirrors were very expensive. And if you broke one, it was reguarly someone important, such as the lord of the kingdom of the king or a high-ranking nobleman. And if you broke it, it was common that if they were un-forgiving, they would put you in jail, possibly for seven years.

Walking under a ladder is seen as bad luck
answer :Typically a ladder means someone at the top of the ladder and that person can easily drop things - onto your head. So it really is a bad idea to walk under one.

Crickets in your home are good luck (not in my home and definitely not for the nasty crickets!)
answer :They tell temperature

Ivy growing on a house protects the inhabitants from witchcraft and evil.
answer :Evil may mistake it for poison ivy and stay away.

Friday the 13th is unlucky
answer :The Templars were all arrested (and most were tortured and executed) one Friday the 13th!