It was in Roman times that the number thirteen became associated with ill omens, particularly those bringing death and destruction. For Christians, 13 represents the number of people present during the last supper. The 13th being Judas who was the last to arrive and the one that betrayed Jesus. Friday is the day that Jesus was crucified. Some believe it to be the day that Eve tempted Adam with the apple.
Today, most hotel chains have no room number thirteen, airplanes avoid the 13th aisle and many skyscrapers are without a thirteenth floor. Check out this National Geographic News article for more about Friday the 13th and the superstitions surrounding it.