i don't know weather this is true, but i heared a interesting version in a movie (he died with a felafel in his hand):
they say that there was an ancient matriarchic culture, that has been forgotten or wiped out of history.
in this culture there were 13 months, what makes sense because the moon circles the earth 13 times a year (364:28=13) also, the female menstruation cycle takes 28 days.
anyway, in this ancient culture, in the 13th month there was a human sacrisfice, usually a man of high status.
but one day a man came to power and changed the system by simply removeing the 13th month to avoid this bloody custom.
so here we are now with 12 months and a patriarchic system and the number 13 considered being unlucky.