Friday the 13th occurs when the thirteenth day of a month falls on a Friday, which superstition holds to be a day of bad luck. Any month's 13th day will fall on a Friday if the month starts on a Sunday.
A BAD-LUCK GUIDE TO FRIDAY THE 13TH - THINGS TO AVOID
Breaking a Mirror: Seven Years of Bad Luck
Putting Your Left Foot on the Floor When You First Wake Up
Walking Under a Ladder
Spilling Salt
Opening an Umbrella Indoors
Leaving Your Purse on the Floor
A Black Cat Crossing Your Path
Dropping a Knife and Picking It Up