As per Niantic’s announcement on Facebook, “Starting the morning of December 25, 2016, and through the afternoon of January 3, 2017 (PST), PokéStops will award one single-use Incubator each day after your first Photo Disc spin. As you visit PokéStops during this time period, there will be a greater chance of finding Eggs that hatch Togepi, Pichu, and several other recently found Pokémon, originally discovered in the Johto region.”
Niantic has also announced that towards the end of the year and into early January 2017, there will be another event that players can expect. “In addition, from the afternoon of December 30, 2016, through the afternoon of January 8, 2017 (PST), the first partner Pokémon and their Evolutions, originally discovered in the Kanto region, are more likely to be encountered. This means a greater chance to catch Bulbasaur, Ivysaur, and Venusaur, as well as Charmander, Charmeleon, and Charizard, plus Squirtle, Wartortle, and Blastoise!”
So if you are looking to increase the number of Pokemon you catch/collect, then those are the dates you’ll want to mark down on your calendar.