You need to use "at Easter". It's a festival, ie. a point in the cycle of a year.
You only use "on" if you mean a particular day, eg. "on Easter Sunday". I've done a quick check using news articles (UK, US, etc), and it's very uncommon to use "on Easter" by itself, if you mean the festival as a time. I'd also consider using "on" in this context as incorrect.