Triskaidekaphobia is the fear of the number 13 and here's what wikipedia has tosay about it:
Thirteen may be considered a "bad" number simply because it is one more than 12, which is a popularly used number in many cultures (possibly due to it being a highly composite number). When a group of 13 objects is divided into two, three, four or six equal groups, there is always one leftover object.
Some Christian traditions have it that at the Last Supper Judas, the disciple who betrayed Jesus, was the 13th to sit at the table, and that for this reason 13 is considered to carry a curse of sorts.
Fear of 13 has also been linked to that fact that a lunisolar calendar must have 13 months in some years, while the solar Gregorian calendar and lunar Islamic calendar always have 12 months in a year.
Triskaidekaphobia may have also affected the Vikings — it is believed that Loki in the Norse pantheon was the 13th god. More specifically, Loki was believed to have engineered the murder of Baldr, and was the 13th guest to arrive at the funeral. This is perhaps related to the superstition that if thirteen people gather, one of them will die in the following year. This was later Christianized in some traditions into saying that Satan was the 13th angel.
Some buildings such as One Canada Square in Canary Wharf (the tallest building in the UK) number their floors so as to skip the thirteenth floor entirely, jumping from the 12th floor to the 14th floor in order to avoid distressing triskaidekaphobics, or the building will have 12th floor and floor 12a or 12b instead. Many hotel/casino megaresorts in Las Vegas also lack a 13th floor. This is often applied to home or hotel room numbers as well, and the same is also true of rows in airplanes, as well as cabins aboard cruise ships
I'd love to hear what happens to people on days like today that they genuinely believe is down to the day thats in it.