Niantic announced the Unova event with all the details but here is a reminder with all the necessary info and a list of Pokemon that will be appearing more often.
Unova celebration event in Pokemon Go will last from Tuesday, January 5, at 10:00 AM to Sunday, January 10, 2021, at 8:00 PM. The even clock is in local time so you don't need to go through any timezone conversions - it will start at 10:00 AM for everyone.
Gen 5 Pokemon from the Unova region will be appearing more often in the wild. These include:
On top of that, the lucky trainers may come across a Shiny Snivy. You can recognise the Shiny version by its cyan colour while the regular Snivy is green.
Meanwhile. hatching the five-kilometre eggs will yield:
You can also start the Unova Collection Challenge before the event kicks off. It should now be live in all regions and it will require you to catch Tepig, Oshawott, Lilipup, Herdier, Zebstrika, Snivy, Solosis, Ferroseed and Roggenrola within seven days.
With all that in mind, it won't be long before the next region is celebrated. Sinnoh event will kick off just two days after Unova concludes , on January 12, 2020.