The Date of Pascha

Why is the Western Pascha celebrated on a different date than the Orthodox Pascha?

The short and quick answer to the question can be summarized as follows:

The most significant change to the calculation of the date of Pascha in the history of Christianity was brought about in the Western Church as a result of the adoption of the Gregorian Calendar in 1582 by Pope Gregory XIII. This took place in order to adjust the discrepancy observed by that time between the paschal cycle approach to calculating Pascha and the available astronomical data. 
The Orthodox Church, however, continues to base its calculations for the date of Pascha on the Julian Calendar, which was in use at the time of the First Ecumenical Council (325 A.D.). As such, it does not take into account the number of days, which have since then accrued due to the progressive loss of time in the Julian calendar.
The change of the calendar in the Western Church caused a major change in the calculation of the date of Pascha with respect to the Eastern Church’s date. It is rare that the two dates coincide. Usually the Western Pascha occurs a week earlier, but sometimes much earlier.
For a longer more in depth explanation see:
Have a blessed Pascha!

Fr. Panayiotis