That's not even the joke. The episode was about Bart doing some higgy ***** raps at a concert and got in trouble or something. Homer (or who the **** ever I dont remember) wanted proof that he was there, so they found a recording of Bart rapping but couldn't prove that he was there at that time. So they fast forwarded and found a rapper that had one of those Flava Flav clock necklaces, but they still didn't have a date. Then they fast forwarded and found that dude who had a necklace that said Thursday the 20th. I wasted a bit too much time on this
This isn't a weed joke. It comes from the Simpsons. It's also a rare occurance for some strange reason. It really will only happen again in 14 months and the last time it happened was 9 months ago.