It's not a government holiday, but I'm pretty sure there are plenty of people who celebrate Easter in some form in Japan.
So some people DO celebrate it in Japan
In Japan, the festival is observed by the Christian faithful. According to a 2000 survey, there were less than two million Japanese Christians of all denominations in Japan, out of a total population of 127 million. Outside the 6,800 or so churches in the country, Easter has little effect on the lives of the vast majority of the population. It passes almost without notice
So some people DO celebrate it in Japan