Thursday the 12th!

Which means tomorrow will be Friday the 13th. Superstition says that means a day of bad luck.

It all got started a long time ago by the Scandinavians. It seems they believed that the number 13 was unlucky due to the mythological 12 demigods being joined by a 13th, an evil one, who brought misfortune upon humans. It was also said that Christ was crucified on Friday and the number of guests at the party of the Last Supper was 13, with the 13th guest being Judas, the traitor.

I found that and other superstitious information at Old Superstitions.