Did you wake up with a foreboding feeling today, like maybe you’ll cross paths with a black cat, break a mirror or encounter a hockey-masked killer? That’s Friday the 13th for you.

Historically, Friday the 13th has seen its share of bad events, including deadly World War II bombings, killer earthquakes and storms and a 1989 stock market plunge. 

But how much do you really know about the unluckiest of all days? Here are five Friday the 13th facts. Maybe they’ll bring a little luck to your day.

1. Scared of the number 13? Then you simply have triskaidekaphobia. But if you’re scared of Friday the 13th itself, you have paraskavedekatriaphobia—also known by the not-really-less-of-a-mouthful friggatriskaidekaphobia.

2. Fear of 13 is nothing new. The Code of Hammurabi—Babylonian law that dates back until at least 1780 BC—omitted a 13th law because it would be unlucky. Theories abound as to why 13’s bad reputation persisted. Some tie it to the number of diners at the Last Supper, or the actions of the 13th dinner guest, Judas. Numerologists also point to 13’s position just after the rounder number 12—12 signs of the zodiac, 12 months in a year, 12 tribes of Israel and so on.

3. So 13 is unlucky, but why is Friday the 13th particularly bad? We probably have a Boston author to thank for that. Friday already was considered unlucky itself, for a host of reasons. But in 1907, Thomas Lawson wrote Friday the Thirteenth, a book that depicts a man bringing down the stock market on that day. An extensive ad campaign helped bridge the separately unlucky 13 and Friday into one massive unlucky day.

4. The pairing of Friday and the 13th is statistically more likely to occur than any other day of the week. For reasons known only to him, mathematician B.H. Brown set out in 1933 to compute the distribution of days of the week on the 13th of the month. The calendar repeats every 4,800 months, or 400 years. Over that time, a Friday the 13th will happen 688 times. That’s more often than the 13th will fall on Sunday (687), Monday (685), Tuesday (685), Wednesday (687), Thursday (684) or Saturday (684).

5. Some years are unluckier than others. There can be as many as 14 months between Friday the 13ths, and every year has one to three. The last Friday the 13th was just in January. Paraskavedekatriaphobics take notice—the next one isn't until September, 2013.