We’re watching the basketball playoffs on TV. Everyone in the audience is wearing a white shirt (or seems to be doing so). How does that happen? Is it real or an illusion?
And who started doing this? And why?
The t-shirts are stadium giveaways. They are either left on the seats, or handed out at the door. Everyone is notified that the game will be a “white-out” (or whatever color), so some people will where white clothing of their own. That’s what’s done for playoff games anyway.
For college games, it’s usually that fans will all wear the school color.
When it happens in a pro game, it’s usually a gimmick to make up for lack of organic enthusiasm among the fan base. (See: bandwagon teams, such as the Heat, or hockey teams that play in the Sun Belt.)
I think they might just hang a white shirt over each chair in the arena, so everyone can put one on. I also think that the Heat started this a few years ago.
1986 Stanley Cup playoffs, Calgary Flames “C of Red” is my guess for the first.
They’re not always giveaways. Sometimes they’ll just announce a “blackout” or “whiteout” or whatever beforehand, and publicize it during ticket sales.
I live in the Miami radio/TV market…(presumably the game you’re watching). There have been fairly constant references to the “White-Hot” Heat. If you’re down here and a Heat fan, you just…know…that they’re calling for a white-out.
I’m not sure if they are giving out t-shirts tonight, but I suspect they are.
-D/a
This is how Penn State does it. It goes on the ticket as a special white out game so everyone knows.
What would they do if you showed up wearing the wrong color? Would they refuse to let you in or would you just look like a dork on tv?
They secretly hold the playoff games in North Korea, where everyone wears what they’re told. 
Wouldn’t there be a big problem with most of them being the wrong size if they’re left on the seats?
I think you either swap with the people around you - or they just make them all one size (say, adult large). A child or small adult can wear an oversize shirt better than a larger person can wear a smaller shirt.
At college games where I’ve gotten free shirts, sometimes they only print a few sizes, or just one size. One day it was just adult medium, large and extra-large. Those t-shirt cannons or thrown-into-the-stands shirts always seem to be size large too.