What is 420?

My birthday is 20th of April. I never knew what 420 was until last year where random kids from my school gave me weed... I didn't really smoke back then, but the point is I never understood why people chose 420 to be the stonersday. Wikipedia says that a group of stoners came together at 4:20 PM to smoke some weed but is there anyone else who knows a different story?

I am really stressing out,because I just don't know WHY

Greets,

Snuffie