Community Event

It will be anywhere. Parks are just a good place to "meet up with other trainers".

It is likely just a temporary change to the game master file causing Pikachu to only be able to learn Surf. That is what they did with Present during the recent holiday event. If so, then we will even be able to charge TM older Pikachus to receive Surf.

EDIT: Here is a source.

https://www.reddit.com/r/TheSilphRoad/comments/7pxiva/introducing_pok%C3%A9mon_go_community_day/dsktugp/?sh=27a36bad&st=JCJW9ZS1

I wouldn’t be surprised if the chance of an ex pass increases if you raid at a park during the event. Just a hunch though.