The thirteenth is actually a lucky number. It represents the number of times the moon cycles through the sky. Just think how much easier it would be to have a calendar of thirteen 28 day months instead of trying to remember which months have thirty-one, thirty, twenty-eight or twenty-nine days.
The fear of 13 is irrational and unsubstantiated.
Just my opinion,
@Erebus, re "...thirteen 28 day months...." Yes it would be a lot easier...though we'd still have to have an extra day somewhere to make up 365, and we'd still have to have leap years.
And @Max, shouldn't they be called Triskaidekaphobes?
Thirteen was rejected by all the fools who embrace "the tyranny of twelve", which makes things go by the dozen when other quantities would be far more convenient. And all of that was done long before we had the metric system to further complicate things.
The Jewish calendar actually does work like that, but it isn't so simple and needs corrections to prevent it drifting away from the seasons. The Moslem calendar is similar, but without the corrections, so it drifts throughout the solar year. Neither are an accepte international standard, so are effectively useless. 13 is not considered unlucky in Judaism, by the way.