Depending on how many people show up, it could be pretty awesome: A Montreal edition of Improv Everywhere's MP3 Experiment is set for Wednesday at 5:17pm.
Instructions are here and there's a Facebook page. No RSVP required, just show up wearing red, yellow, blue or green and carrying an uninflated red or green balloon to a retail store on Ste. Catherine St. between St. Alexandre and St. Laurent.
Improv Everywhere's MP3 experiments are explained here, but in short they involve a large group of people downloading an MP3 audio file, then playing it on headphones at exactly the same time. To the outside world, it just looks like a bunch of people wearing headphones, but who suddenly all start doing the same random things together.
Montreal had a similar thing in 2009 that I participated in, and although there were some glitches it was a lot of fun for those involved.
The weather forecast is for light showers with a high of 17 degrees.
UPDATE: Just For Laughs has an announcement set for 5:30pm. Since JFL is organizing the MP3 experiment as well, I'm guessing the timing is related.
UPDATE (May 11): The event went well, with just about every radio and TV station sending people (for both the JFL lineup announcement and the MP3 experiment). It beat the rain by about an hour.