Today is Friday the 13th.
But I think 13 is a lucky number. There are 13 full moons in a year. In the pre-Christian calendar of Europe there were 13 months of 28 days. But there was an extra day, this was Halloween. Since it was an extra day, it was a day between the worlds. As such it was a day were you could talk to the dead. The spirits of those who had passed on were said to be closer. It was also their New Years Day. It was a day when the normal rules did not apply. So, 13 is about the cycles of the moon and the earth. Of course it is lucky.