The mirror has been a source of mystery for as long as humans have been alive. More than just a surface from which you can check your hair from, the mirror has traditionally been associated with sorcery and magic.
From Snow White's evil stepmother to China Mieville's The Tain, the mirror has been used as a vehicle for things not good, its reflective surface, once benign, now used for human destruction.
It's not surprising that stories and urban legends have grown around mirrors as well. Some of the more popular ones include child murderer Bloody Mary (say her name five times in front of a mirror); The Candyman, the movie based on Clive Barker's short story The Forbidden (say his name three times), and on a local note, Nick Joaquin's May Day Eve, where chanting "Mirror, mirror, show to me him whose woman I will be," will either show you the person you are to marry or the devil himself.
May Day Eve may not be a horror story (not in the traditional sense, anyway), but it does make use of something that everyone can relate to: an element of fear.
Eleven days to go til Halloween, people. Trick or treat!