Pokemon Go's last event of 2016 ran December 12, 2016 through December 29, 2016.
The Johto Holiday Update brought with it several new Pokemon including Pikachu wearing a festive hat.
Trainers could catch the holiday Pikachu like any other Pokemon. Niantic announed that the Pikachu in the holiday hat will have a higher frequency rate than normal Pikachu. The special Pikachu was available from December 12 to December 29 at 11AM PT. If you evolve the Pikachu with the hat, the Raichu keeps the hat.
The other new Johto Pokemon don't have an event end date, but unlike the Pikachu, you can't catch them. You have to hatch them. So far six new Johto baby Pokemon have been confirmed with one new evolution.
Eggs are still being confirmed, but these distances have been collected by TheSilphRoad Reddit.
Once you catch or attempt to catch a holiday Pikachu you'll be able to go to your Pokedex and view the baby Pokemon silhouettes when looking at the evolved forms.