For some cultures, it's not Friday the 13th that they're worried about. It's Tuesday.

Greece
Tuesday is considered, by many Greeks, to be the unluckiest day of the week. It was on Tuesday, May 29th, 1453 that the city of Constantinople was besieged and taken by the Ottoman Turks. The loss of life was considerable.

Conversely, Greeks consider the number 13 (on its own) to be good luck. Reasons for this vary, including the belief that having 12 apostles of Christ made Christ the 13th of the group. The ancient philosophy of Numerology considers 13 to be "the most selfless of all" and "love for the world revolves around" thirteen. The Celts also considered the number 13 to be related to good fortune.

However, the combination of Tuesday and 13 as Tuesday the 13th of the month is considered a very unlucky day in Greek culture.

Mexico, Spain and Latin America
Spanish speaking cultures fear "Martes Trece", Tuesday the 13th. It's bad luck to have a wedding or board a ship on a Tuesday.

"Martes" comes from the word Mars, the Roman god of war. And war is widely considered a bad or unlucky thing to happen to a person.

The 13 reference goes back to Christianity again, but this time it refers to Christ in a bad way as the 13th in the group with 12 apostles. 13 is also linked to Judas, the 13th apostle.

Another reference notes that the Antichrist appears in chapter 13 of the Apocalypse in the Bible, and that Adam ate the apple on a Tuesday.

Sources
http://www.greekspider.com/superstitions/greek_superstitions3.htm
http://www.greekspider.com/superstitions/greek_superstitions4.htm
http://www.astro.gr/Rabi'a/arithen.htm
http://www.whittierdailynews.com/