Hello Pokemon Fans! Today is the start of the annual Winter Event! In this event, you’ll notice many new changes and an exciting, highly-anticipated event!
This event will run from December 23rd, 2017 until January 6th, 2018. When the event ends, Zygarde will not be obtainable until the next event. Make sure you finish before then!
Below is the change-log for today’s update
Added Zygarde Event
For a limited time, you’ll be able to collect up to 100 Zygarde Cells and 5 Zygarde Cores! Once you get 10 cells, you unlock Zygarde’s 10% form. 50 cells unlocks Zygarde’s 50% form, and 100 cells unlocks Zygarde’s Complete form. Obtaining Zygarde Cores unlocks exclusive moves for Zygarde at the Move Tutor!
Zygarde Cores are specially hidden in the game and can be earned through several different methods.
Core 1: Rarely hidden in Presents
Core 2: Sometimes found when fighting other players
Core 3: Hidden high up on the map
Core 4: This one’s a freebie. Type “Z1Z2” in the chat to unlock it
Core 5: Hidden deep down within the map
Added Pokeboxes
Pokeboxes contain various cosmetic skins to use with your Pokemon. These skins only change how the Pokemon looks, they do not change their stats. Pokeboxes can be purchased and opened in the town.
Added new Pokemon:
Stufful, Bewear
Added new Moves:
Sheer Cold, Frost Breath
Added support for 3 new code Pokemon
These 3 code Pokemon will be released as free global codes for everybody on certain days during this Winter Event. Make sure to keep an eye out so that you can get them all!
Optimized Bag to remove lag spikes in certain scenarios
Optimized Quest Log to reduce remote requests & lag
Fixed an issue allowing pokemon to gain double speed
Fixed an issue causing players to not “Explore the Underwater dimension”
Fixed an issue where Ditto couldn’t transform into Deerling/Sawsbuck
Daily Task rewards have been buffed from 10/15/20/25 to 25/50/75/100
Same tasks as before, but you get more Pokebux!
Edit:
Zygarde Cells can spawn everywhere on the map. One cell spawns every 15 seconds and despawns after 10 minutes. They may seem tough to find, but that’s because there are so many different places they can be! 
Edit 2:
This event has now ended.