I don't go in for superstitions of this sort. I've never had any trouble with anything that is supposed to cause luck, good or bad, and to me the whole idea is silly.
It all has to do with the way human brains recognize patterns, sometimes seeing ones that happen purely by accident. And when you're expecting bad luck (say, because it's Friday the 13th), you'll notice the negative events in your day much more acutely, and thus the belief gets reinforced.
Besides, if events, objects, or dates were truly bad or good luck, then the beliefs concerning them would be the same across every culture - and they're not.
Okay, lecture over.