See this excerpt on Friday the 13th:

It's strange how society has come to accept "superstition" so plainly and unquestioning. Go to every hotel in the States and there is almost never a floor with the number 13 in it. However, if you should venture to a hotel in Italy, you will indeed find that 13 is not skipped over. In fact, a gold charm of the number is given to Italian infants to ensure good luck and prosperity in their developing life. Why then, is there such suspicion surrounding 13? Furthermore, ever wonder what makes Friday the 13th so "evil"? (Note: Friday the 13th was considered a day of bad luck and evil long before the movies came out!)

There are various theories surrounding 13. Since Christianity has had a extremely large influence on all societies as a whole, we shall look there first. At the "Final Supper" or "Last Supper" (whoever you chose to term it) there were 13 guests. Some believe Jesus to be the 13th at feast, occurring just before he was Crucified. This is a largely debatable theory, however. How does one know where to start counting? Many chose to go by the painting "The Last Supper," forgetting that this is an artist's interpretation of an event he did not attend. The painting depicts the disciples and Jesus at a table, but evidence suggests that the last supper was not conducted at a table. Rather the participants were seated on the ground where it was cooler. (The climate in Israel, at that time, suggests they would have stayed on the ground to keep cool.) Furthermore, where to start counting (when assuming the picture is accurate)? If one started with Judas, Jesus became the 13th guest. Conversely, if the count began at Jesus, it was indeed Judas who was 13. Both options would contribute to the mystique surrounding 13: Jesus, for he was killed soon after the feast, Judas for he was the betrayer of Jesus."

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Here's more from that website: Friday the 13th