Paraskevidekatriaphobia: A morbid, irrational fear of Friday the 13th. Also known as friggatriskaidekaphobia.
I'm not really sure how some people can be afraid of a day. I mean, it's just a day. If you're going to be afraid of something, make it something genuinely creepy, like clowns or soft sculpture mannequins.

Look at those things. They look like shriveled up dead people. Yeesh.