Double candies during Helloween event 2017
During this event when I transfer Pokémon or evolve it I will get double candies, but is this true for all situations when I can get candies? For example, does it work when I hatch an egg? Will I receive double number of candies for it? Or when I feed Pokémon in a gym, will I get 2 candies sometimes? Or is some another bonus which I did not notice yet? Now I know only when I use Nanab berry I will get a quadruple candies.
From Niantic's own update:
You’ll also earn extra Candy while you’re out trick-or-treating with your Buddy Pokémon: Candy rewards from catching, hatching, and transferring Pokémon will be doubled, and your buddy will find Candy twice as fast during the Halloween celebration!
(emphasis mine). So probably you will not get double candy for giving a berry at a gym, but the other you will get double candies for.