Needless to say it is not only Spaniards that consider this day unlucky, but Greeks as well.
Another reason that thirteen is considered to be an unlucky number in so many countries comes from The Last Supper, when Jesus gathered this 12 disciples together for what turned out to be their final meal together. 12 apostles + Jesus himself = 13. As Jesus was betrayed by the apostle Judas and was subsequently captured by the Romans and crucified, it was subsequently considered unluckiy in many Christian countries to have thirteen people together.
Friday - while being many people's favourite day of the week these days - becomes unlucky when coupled with the number 13 again thanks to the Last Supper and Jesus' crucifixion.. the Bible states that the son of god was flogged, forced to carry his own cross and then was nailed to it and left to die. On the day we celebrate as Good Friday! Some believe the name Good Friday is a corruption of "God's Friday" - or that the word "good" actually meant "holy" - as the death of Jesus Christ hardly seems something Christians would consider good!
Of course all this talk of bad luck, whether on Friday, Tuesday, the 13th or the 17th naturally leads us to talk of superstition. Even in our relatively secular and theoretically rational society there are still people who share a number of popular superstitions, such as not walking under ladders, trying not to spill salt and not opening an umbrella while still indoors.