Taken from Public Orthodoxy…..
There is a common misperception among Orthodox Christians that the reason why Orthodox Easter (i.e. Pascha) often occurs so much later than Western Christian Easter is because the Orthodox Church abides by the rules for calculating the date of Pascha issued by the 1stEcumenical Council at Nicaea in 325 AD and thus the Orthodox must wait for Passover to be celebrated by the Jewish community before Pascha can occur. Despite this view being held by so many Orthodox Christians as well as being promoted in popular essays written by some Orthodox priests, it is not accurate. The reason why Orthodox Pascha frequently occurs so much later than Easter celebrated by Roman Catholics and Protestants has nothing to do with the Orthodox Church following the Paschal formula of Nicaea and the Western Churches not doing so, nor is it because the Orthodox must wait for Jewish Passover to be celebrated. Rather, Orthodox Pascha frequently occurs later than Western Easter because the Orthodox Church uses inaccurate scientific calculations that rely on the inaccurate Julian Calendar to determine the date of Pascha for each year. Some background information is in order to help explain precisely what the problems are. Continue here