Tomorrow, on July 22nd, a global Pokemon Go event will begin. The goal of the event will be to catch a given, presumably large, amount of Pokemon. If enough pocket monsters are caught (sounds dirty), the first Legendary Pokemon ever will be revealed in the Grand Park, Chicago. If that beast will be defeated by the end of the day, it will begin to appear in Pokemon Raids even after the Go Fest is over.
If you cannot attend the event for one reason or another (for example you do not live in Chicago), you can watch it being streamed live on YouTube.