1. The Templars' Curse
The Order of the Knight Templars was annihilated on Friday, October 13th 1307. It was on this day that the Pope Clement V of the Roman Catholic Church, with the help of Philip IV, King of France, arrested and sentenced a powerful monastic military order known as the Knights Templar to death. Philip IV ordered the torture and crucifixion of their leader.
However, because of the political and financial Power of the Order, the Day was considered a cursed day by all of their followers and this is how it remained in history.
2. The Last Supper
Christians consider "bad luck" to gather 13 people to eat as 12 were the Jesus Christ's disciples + Himself who ate at the "Secret Supper" which took place right before Jesus was arrested. Jesus died on the Cross on Good Friday. This is why many Christians consider Friday the 13th, bad luck.
3. Ragnarok, the "doom of the Gods"
Similarly to the Christians' Last Supper, there is a Norse Myth according to which, 12 Gods where sitting when Loki - the trickster appeared. This leads to a series of unfortunate events and the death of Baldur, a beloved God. The story portrays the archetype of the Cosmic Cycle of Birth, Life, Death & Rebirth which apparently is similar for every scale of existence, even Gods!
4. Olympus can have ONLY 12 Gods
It seems like the blessed Greek Mountain of Olympus has only seats for 12 Gods. According to the Ancient Greek religion, 12 where the Gods of Olympus right before Hermes (Mercury in Latin) birth. But when Hermes, the God of Communication was born, Hades, the God of the Underworld, left Olympus to Maintain balance of the Divine Realm. This is why Zeus, the mighty King of the Gods, promises to his brother to give blessings so that Hades could marry Persephone and live happily in the Underworld.
The number 12 is considered one of the most potent and perfect numbers (this is why the Months are 12, the Gods of Olympus are 12 etc). The number 13 equals 4 according to numerology, a number of Mars, which can bring anxiety and accidents.
5. Apollo 13, the unsuccessful
The Apollo 13 was the 7th manned mission in the American Apollo space program and the third intended to land on the Moon and it was the 1st unsuccessful mission of NASA (at least for the masses). The craft was launched on April 11, 1970.