Trainers, this is just in!
Niantic Support confirmed that stamp must be collected after May 1, 1:00 PM PDT to encounter Zapdos.
As long as your last Stamp to achieve a Research Breakthrough is earned after May 1 1:00 P.M PDT, you’ll have the opportunity to encounter Zapdos! https://t.co/PX61ktGBZZ
— Niantic Support (@NianticHelp) April 30, 2018
Good luck and catch ’em all!