Truth Behind The Easter Celebration

How is the event date picked and for what reason does it change each year?
Fundamentally, the dates for Christmas, Valentine's Day and Halloween are always settled, but Easter can fall at whatever week between March 22 to April 25. Do you know why?
Because the celebration of Easter depends on the moon methodology. The full moon included isn't galactic Full Moon, but a clerical moon (chose from tables) that keeps, essentially, in a stage with the Cosmic Moon.