The choice between "at Easter" and "on Easter" depends on the context and what you're trying to convey.
At Easter
It refers to the general period of time around the Easter holiday, not just the specific day. It is used when talking about activities, events, or traditions that happen during the broader Easter season, not necessarily on the specific day of Easter Sunday.
The school is closed at Easter.
We usually visit our grandparents at Easter.
We always travel to the countryside at Easter.
At Easter, many people go to church services.
On Easter
It refers specifically to Easter Sunday, the day of the Easter celebration. 'On Easter' is used in informal language. 'On Easter day' or 'on Easter Sunday' is more used than 'on Easter'.
We go to church on Easter.
I’m planning a family dinner on Easter Sunday.
We went for a long walk in the park on Easter day.
Key Difference
"At Easter" = the broader Easter period (include multiple days).
"On Easter" = specifically Easter Sunday.