The ecclesiastical rules are:

resulting in that Easter can never occur before March 22 or later than April 25.

The date of Easter is primarily used for liturgical purposes. Up to the eighth century there was no uniform method for determining the date of Easter but the method favoured by the Council of Nicaea (present-day Turkey). in (325 AD) gradually became the accepted method.  The current Gregorian ecclesiastical rules that determine the date of Easter trace back to the First Council of Nicaea convened by the Roman Emperor Constantine. At that time the Roman world used the Julian Calendar (put in place by Julius Caesar (46 B.C.) . Prior to the Council of Nicea , Christians in general would celebrate Easter on the 14 Nissan, whichever day that was, which is when Jews celebrate the Passover but it was felt that such an important holiday should be separated from this high holy Jewish day.

Although the Julian or solar-based calendar of the Roman Empire had been in place since 45 B.C., it did not supplant the lunar calendar that was the chart and compass of 2,000 years of Jewish history. (A lunar year is 12 lunar cycles of 29.53 days each or 354.36 days while a Julian year is 365.25 days with a leap day every four years.) The Julian calendar functions by having three years of 365 days and one year of 366 days every four years.

Lets see if I can put this in simple terms ( I aint  promising!)

 

Its basically because we are using a different system to the one that folks back then used... they went on the moon cycles similar to what muslims use today (e.g Ramadam which is the equivelent to our Lent is a different time each year).. We changed ..  so theoretically by their calandar it is the same date every year just on ours it isnt..

 

 

 

 

Year:	2008	2009	2010	2011	2012	2013	2014
Day	23	12	04	24	08	31	20
	March	April	April	April	April	March	April

rofl.. well ya did ask