It is well known that Greek-Orthodox Christians get ready to celebrate Easter on Sunday the 1st of May. There are reasons that explain why Greek-Orthodox Christians celebrate the Resurrection of Jesus at a later date than Catholics do. The explanation is slightly complicated but can be simplified and broken down. The difference mainly occurs due to the calendar and the obedience of the Orthodox Catholics concerning the early practices of the Christian Church.
Firstly, in relation to the calendar element, the Christian Orthodox Church continues to adhere to the Julian calendar when counting for the date of Easter. In contrast, other Christians follow the Gregorian calendar. The calendars' dates differ by 13 days.
Moreover, the Orthodox Church adheres to the rules laid out by the First Ecumenical Council, which was decided in 325 AD in Nicea. The council ruled that Easter (Pascha) must take place following the Jewish Passover in order to maintain the sequence relating to Christ's Passion. Other Christians are slightly ignorant of this factor and thus follow different dates. This results lead them celebrating Pascha either before or after the Orthodox Church.
So, the two issues above explain why the Orthodox Church often celebrates Pascha at a later date than Western Churches do. The timing can vary from one to five weeks' difference. At times, both Churches can celebrate Easter on the same day depending on the ways that the calendars fall. This was the case in 2011.
The two Churches dates are the same when the full moon, after the equinox arrives so late that it is considered as the first full moon after March 21st in both calendars. However, this is a rarity but has occurred more frequently in recent years (2010, 2011, 2014 and it will again in 2017).
This causes particular frustration and confusion for many. This is particularly the case for Greek couples that are looking to set a date for their wedding and for Greek families who do not follow Orthodox traditions. They are left wondering which date to set their special day for and why they have to celebrate such a sacred occasion (Pascha) at different times.