For me it would have been in '92 during Comedy Central's Thanksgiving marathon.
Both my sister and I were home from college for the weekend, and I just happened to by flipping through the channels on the tiny 12-inch screen I had in my old bedroom, when I found it. I was laughing so hard my sister had to find out what I was watching. Next thing we know, the two of us had been sitting there all day watching.
I knew nothing of the show until 1996 when the movie came out. My older brother was a fan of the show so he dragged me to a theater in Dallas that was playing it. I laughed so hard I could barely speak the rest of the night. The rest is history.