Easter is a festival celebrated by Christians globally as a day of resurrection of Jesus Christ. It is one of the important festivals for Christians, and it is also known as a moveable feast. They call Easter a moveable feast as it does not come on the same date as date of the festival changes every year, like other holidays celebrated by Christians.
However, Sunday is the day of the Easter, which was fixed by Bishops centuries ago, so people started celebrating only on Sunday following by the full moon after the vernal equinox that is between March 21 and April 25 the festival will be celebrated. The traditional Christians will calculate the day of the Easter using the Julian calendar, so they celebrate the festival after a week or two weeks, but Western Churches follow the Gregorian calendar and so they celebrate the holiday accordingly.