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1.
Having a killer blaze session with a bong.
Saul: You still got that bong I got you when I was in Tel Aviv?
Red: Hell yeah, Bong Mitzvah! Hit it up, dude!

-Pineapple Express
by Zach The Magical Hobo July 19, 2009
 
2.
On the first Saturday (Shabbat) following 13 days after purchasing a new bong, the bong will go through it's Bong Mitzvah. The celebration is meant to welcome the new bong into the family. Traditionally at least 10 people (a minyon) wearing yarmulkes and talit should be present to take part in the festivities.
Hey man, your bong is 13 days old? What time is the Bong Mitzvah on Saturday?
by Bobby Gizzle November 06, 2010
 
3.
A gathering with just a couple of people or a whole party that are there to smoke weed.
"Hey i was thinking, bongmitzvah this weekend?"

"Yeah!! But only like 4 or less? Last time there was too big of a head count."
by meggiedough November 15, 2009