I`m sorry to contradict you but the expression at Easter is correct:
We have to use AT with :
(1) clock times (EX) at six o'clock
(2) religious festivals (EX) at Christmas, at Easter [meaning at Christmas time / at Easter time]
(3) mealtimes (EX) at breakfast, at lunch, at dinner, at dinner time
(4) other specific time (EX) at night, at the weekend , at weekends, at half-term
On Easter means exactly on Easter Day.
At is used for specific points of time not only for place or location.
at night The stars shine at night.
at the weekend I don't usually work at the weekend.
at Christmas/Easter I stay with my family at Christmas.
at the same time We finished the test at the same time.
at present He's not home at present. Try later.
All the best.
- For Teachers