Yay! Exclusive periods of events, so it's like Splatoon 2 Splatfests!Some more information about how the Easter event works:
The information about the event is delivered via BCAT (Background Content Asymmetric synchronized delivery and Transmission), it's a service provided by the Switch OS that downloads and updates game data in the background.
It's basically just a flag, Nintendo sets it to enabled then disable it whenever they want. They could enable the event any time of the year.
The game also checks that you have the update 1.1.1a installed before enabling the event (this update hasn't been released yet).
This means that the events won't be permanently enabled for future years, they will always require internet connection and Nintendo to enable them manually.
Also the start times differ depending on the region you set your console to.