Breaking a Mirror Brings Seven Years of Bad Luck
"The superstition that shattering a mirror curses the person responsible with seven years of misfortune."
Traces to ancient Rome, where mirrors weren't thought to simply reflect a face, they were believed to capture a piece of the soul itself. Breaking one meant damaging that soul-image, and Romans also believed the body and soul underwent a complete renewal every seven years, giving the exact length of the supposed bad luck. There's a more practical layer too: mirrors were extremely expensive for most of history, and breaking one could genuinely cost close to seven years' worth of an ordinary person's wages, which may have reinforced the superstition long after anyone remembered the original Roman soul-belief. The superstition spread far more widely once mirrors became mass-produced and affordable in the late 1800s, putting them into ordinary households for the first time.