So... I'll put it all together now.
Most of you guys don't believe in Friday, the 13th as a cursed day much and only few of you could tell about bad luck on this day (Soshi Kitai and AngelFlow).
In this thread we also found out something about why the number 13 was cursed (thank you Chubz for that) and we got to know about other cursed numbers too (4 in China and 666).
Thank you for your posts, now I'm sure that I'm not the only one who doesn't believe in this. Have a nice day!