If you have a superstitious dread of the number 13 (a condition technically known as "triskaidekaphobia") then this will be a bad year for you. Unlike the floors in multi-story buildings, we can't just skip 2013 and call it 2014 instead. But you can count your luck that there isn't a thirteenth month, otherwise you would have to suffer through 13/13/13!
2013: The year of triskaidekaphobia
Do you believe in unlucky 13, or lucky 13?
There are a lot of reasons given for why Western cultures consider the number 13 to be unlucky. But you can find these patterns throughout history if you care to look for them. You could just as easily amass a damning collection of facts about the number 11 or 8. And most other cultures have their own unlucky numbers. (In Asian countries, 4 is the equivalent of 13 as a number to be feared and many multi-storied buildings lack a 4th floor.)
Perhaps the most substantial connection between 13 and bad luck is that there were 13 people seated at the Last Supper. This is why dinner party hosts are urged to never have 13 people at the same table. Judas was the last person to be seated, the 13th guest, and look how that turned out.
Why is Friday the 13th particularly bad news? (I mean, aside from being the title of a long-running horror movie franchise of varying quality.) One theory holds that Fridays were considered bad luck because in medieval Europe, that's when the hangmen plied their trade. Add that to the unlucky number 13 and you have an all-purpose bad luck day, for no real reason.
(By the way, there is only one Friday the 13th in 2013. It falls in September.)
Of course, 13 is also considered a lucky number. In fact, in a highly unscientific survey which I just conducted at Googlefight.com, "lucky 13" beats out "unlucky 13" by a wide margin. You could argue that this is because Lucky 13 is the name of a popular clothing manufacturer. But I say it's all just a matter of cause and effect. The clothing manufacturer obviously took its name from the idea of 13 being lucky, not the other way around.
According to speculation on Wikipedia, 13's powerful reputation may be due to many cultures following a lunar calendar in which a year has 12 normal months and then one smaller "often portentous" thirteenth month. It's as good an explanation as any.