Today is 4/20, better known to pot heads everywhere as The Stoner New Year. What you may ask, is the origin, of 4:20, a key number in stoner culture? The answer:

We were too stoned to remember.

Wikipedia has a pretty good theory though:

Snopes.com, High Times magazine, The Marijuana-Logues, and The Straight Dope claim that in the early 1970s, a group of teenagers at San Rafael High School in San Rafael, California used to meet every day after school at 4:20 p.m. to smoke marijuana at the water tower. One piece of evidence supporting an origin of the term from the time 4:20 is the fact that the number is always said “four twenty”. This theory is also the most cited, and the most widely-accepted.

So there you go. Totally meaningless. Just like half the things said while high.