Per Niantic, those in Grant Park can unlock rewards (like double stardust) for everyone globally, while we outside can extend the length of these rewards (like for more days). If we all — inside and outside — win our challenges, then the mystery challenge will unlock. Everyone believes that the mystery challenge is legendary raid bosses! The only question is which.
(Golem with 2 rock moves is the go-to counter for all the legendary birds, as Tyranitar with 2 dark moves is for Mew and MewTwo. If you don't have those, try Omastar and Houndoom, or Sudowoodo and Gengar.)
During the worldwide challenge windows, if you can, go out and catch! Every capture counts. Check your time zone on GoHub. In Pacific Time, the windows are:
- Challenge #1 9:00 AM to 9:30 AM
- Challenge #2 11:00 AM to 11:30 AM
- Challenge #3 1:00 PM to 1:30 PM
(Sources: TrainerTips, Mystic7, Reversal, ProdigiesNation, GoHub, The Silph Road, GamePress, PokéAssistant, Niantic, Forbes...)