The origins of the connection between the number thirteen and ill fortune are similarly obscure. Many different sources for the superstition surrounding the number thirteen have been posited, the most common stemming from another Christian source, the Last Supper, at which Judas Iscariot was said to have been the thirteenth guest to sit at the table. (Judas later betrayed Jesus, leading to His crucifixion, and then took his own life.) This Christian symbolism is reflected in early Western references to thirteen as an omen of bad fortune, which generally started to appear in the early 18th century and warned that thirteen people sitting down to a meal together presaged that one of them would die within the year:
* "I have known, and now know, persons in genteel life who did, and do, not sit down to table unmoved with twelve others. Our notion is that one of the thirteen so partaking, will die ere the expiry of the year." (1823)
* "The old story runs, that the last individual of the thirteen who takes a seat has the greatest chance of being the 'doomed one'." (1839)
Superstition held that the victim would be the first person to rise from the table (or the last one to be seated), leading to the remedies of having all guests sit and stand at the same time, or seating one or more guests at a separate table:
* " . . . Miss Mellon always gave the last comer an equal chance with the rest for life . . . she used to rise and say, 'I will not have any friend of mine sit down as the thirteenth; you must all rise, and we will then sit down again together.'" (1839)
* "Every one knows that to sit down thirteen at a table is a most unlucky omen, sure to be followed by the death of one of the party within the year . . . Some say, however, that the evil will only befall the first who leaves the table, and may be averted if the whole company are careful to rise from their seats at the same moment." (1883)
* " . . . so far is this feeling carried that one of the thirteen is requested to dine at a side table!" (1823)
(The "thirteen at the table" form of superstition again harkens back to the Last Supper: the one who left the table first, Judas Iscariot, died at his own hand soon afterwards.)
More generally, groups of thirteen people in any context — at a table, in a room, on a ship — were believed to inevitably lead to tragedy:
* "On a sudden an old woman unluckily observed there were thirteen of us in company. This remark struck a panic terror into several who were present . . . but a friend of mine, taking notice that one of our female companions was big with child, affirmed there were fourteen in the room . . ." (1711)
* "Notwithstanding . . . opinions in favour of odd numbers, the number thirteen is considered as extremely ominous; it being held that, when thirteen persons meet in a room, one of them will die within the year." (1787)
* "Many will not sail on a vessel when [thirteen] is the number of persons on board; and it is believed that some fatal accident must befall one of them." (1808)
By the late 19th century the superstition surrounding thirteen had become even more general, with people going out of their ways to avoid anything designated by the number thirteen, whether it be hotel rooms, desks, or cars:
* "'Look at that,' said Parnell, pointing to the number on his door. It was No. 13! 'What a room to give me!'" (1893)
* "For some time before the late War I went almost daily to the British Museum reading room . . . I gave some attention to the desks left to the last comers . . . there was a very marked preference of any other desk to that numbered '13'." (1927)
* "The mechanic helped him get out [of the racing car]. 'May as well scratch,' he said. 'He won't be good for anything more this afternoon. It's asking for trouble having a No. 13.'" (1930)
Once again these ill omens were avoided through artifice, such as the renumbering of rooms in hotels and inns to eliminate any Room #13's, and misnumbering the floors above the 12th floor in multi-story buildings so that tenants could pretend 13th floors were really 14th floors.
http://www.snopes.com/luck/friday13.asp