No theirs a different reason see,

XIXI is an omen of bad luck. In fact, in Italy, 13 is generally considered a lucky number. However, due to Americanization, young people consider Friday the 13th unlucky as well.

BECAUSE 13 IS THE AGE OF TEEN SO U START TO GROW BUMBS RED SPOTS ETC ETC ITS LIKE A NIGHTMARE!

OH WAIT I KNOW WHY

Ikr it's full of shiiiiiiiiiit

unlucky day i guess