They are celebrating all the Community Day that came before it, and it’s a lot to process given how many Pokémon are appearing and which ones are going where. There are two “traditional” Community Day three hour periods, one on Saturday, December 14 (today) and one on Sunday, December 15 from 11 AM to 2 PM local time. If you’re going to be hunting for shinies, you should probably know what you’re getting and where those will come from. Sunday (11 AM - 2 PM): Mudkip, Ralts, Trapinch, Turtwig, and Chimchar will appear more in the wild. These are past Community Day Pokémon and they should have the same shiny rate as the others, if not higher.

Source: Forbes December 14, 2019 12:38 UTC