What happens for Winter holidays in LoL?

Usually all or most of the winter skins get re-released. Last year, you could gift friends summoner icons that cost 100 IP each. They were the cute little penguins and snowballs.

This year, I believe they're giving summoners rift a "skin" so that it looks winterized. Likely 2-3 new winter skins, new winter ward skins.

As for event, it varies every year, but I'm sure there will be one.

Something similar to what you saw for harrowing with the addition of the map being covered in snow.