On Friday, October 13, 1307, officers of King Philip IV of France arrested hundreds of the Knights Templar, a powerful religious and military order formed in the 12th century for the defense of the Holy Land. Imprisoned on charges of various illegal behaviors (but really because the king wanted access to their financial resources), many Templars were later executed. Some cite the link with the Templars as the origin of the Friday the 13th superstition, but like many legends involving the Templars and their history, the truth remains murky.
In Germany, according to an old tradition, 13 (dreizehn), as the first compound number, was the first number written in digits; the numbers 0 (null) through 12 (zwölf) were spelled out. The Duden (the German standard dictionary) now calls this tradition (which was actually never written down as an official rule) outdated and no longer valid, but many writers still follow it.