Why Easter has different dates?
How to calculate Easter?
How do you determine Easter?
Spis treści
How do you calculate Easter?
Setting the date for Easter. Why Easter is a moveable holiday? Why Easter have different dates? How do you determine Easter?
As we already know Easter in the calendar is a movable feast. This was established during the first Council of Nicea, which took place in 325. It was then determined that Easter Day would be celebrated on the first Sunday after the first spring full moon. The very model for setting this date at the First Council came from the principles of translating a specific date in the religious Hebrew calendar (the date 14 nisan  nisan is the name of the month), into the solar calendar, the Julian calendar. The date 14 nisan represents the beginning of the Passover festival in the Hebrew calendar.
It therefore follows that Easter is a movable holiday. Easter Day itself may fall on March 22 at the earliest and April 25 at the latest. Other holidays during this period associated with Easter are also movable. Corpus Christi, Pentecost (specifically 50 days after Easter), Ascension of the Lord, the whole Paschal Triduum (Maundy Thursday, Good Friday, Holy Saturday, Sunday), Ash Wednesday or finally the period of Lent. Some changes occurred after the introduction of the Gregorian calendar. However, they did not apply to the Orthodox faith, which continues to celebrate holidays in accordance with the Julian calendar.
In history Churchand there have been attempts to standardize Easter and set a specific date for it.
Such an attempt was made under Pius X. The draft came close to being adopted, with the majority of bishops agreeing, but eventually, after protests accusing attempts to diminish the importance of the resurrection of the Lord, the changes were rejected.
So far, there have been those who have advocated changing the dates for celebrating Easter. They are talking about choosing always the second Sunday of April, which would give dates from 8 to 14 April or choosing Sundays between the Feast of Epiphany a Ash Wednesdaywhich would also coincide with the dates 814 (except in leap years, when it would be April 713). It is perfectly acceptable for the Church to change the date.
CALCULATIONS
How do you calculate Easter in a given year?
When setting the date of Easter, it is important to keep in mind the basic rule that states Easter always falls on the first Sunday after the first spring full moon occurring after March 21. Thus it falls between March 22 and April 25. The method of calculating the specific day of Easter was given by the German mathematician C.F. Gauss. The table given below is used in this case:
Years 
A 
B 
Exceptions (year) 

I kind of 
II kind 

331582 
15 
6 
no 
no 
15831699 
22 
2 
1609 
no 
17001799 
23 
3 
no 
no 
18001899 
23 
4 
no 
no 
19002099 
24 
5 
1981, 2076 
1954, 2049 
21002199 
24 
6 
2133 
2106 
22002299 
25 
0 
2201, 2296 
no 
23002399 
26 
1 
no 
no 
24002499 
25 
1 
2448 
no 
25002599 
26 
2 
no 
no 
26002699 
27 
3 
2668 
no 
27002899 
27 
4 
2725, 2820 
no 
29002999 
28 
5 
no 
no 
We use 6 steps for the calculation:
 divide the number of the year by 19 and then find the remainder a
 divide the number of the year by 4 and then find the remainder b
 divide the number of the year by 7 and then find the remainder c
 the remainder determined as a, multiply by 19, add the number A to the product, divide the sum by 30 and determine the remainder d
 Sum of products of 2b + 4c +6d + B divide by 7 and determine the remainder e
 Sum of the residuals e + d add to date March 22
This is how we get the date of Easter.
If the date is above March 31, it must be converted to the corresponding day in April. If the sum (d+e)>10 then we are sure that it is April. In the opposite case it is March.
Exceptions to the Calculation of Easter Dates
In some cases there are also exceptions (first type exception, second type exception)
Exceptions of the first kind occur when d=29, a e=6 and according to the calculations Easter would fall on 26 April. In that case, it is celebrated a week earlier, on April 19 (e.g. 1609 and 1981).
Exceptions of the second kind occur when d=28 a e=6, and dividing 11A + 11 by 30 gives a remainder less than 19. In that case Easter falls on April 5, and is celebrated on March 29 (1807 and 1954).
All exceptions that occur up to the year 2999 are listed in the table above.
EXAMPLE:
Let's calculate the date of Easter for the year 2008;
 Step 1: 2008:19=105, remainder a=13
 Step 2: 2008:4=502, remainder b=0
 Step 3: 2008:7=286, remainder c=6
 Step 4: (19+a+A):30 or (19+13+24):30=271:30=9, remainder d=1
 Step 5: (2xb + 4xc + 6xd + B):7 i.e. (2×0 + 4×6 + 6×1 + 5):7=5, remainder 0
 Step 6: d+e+22 or 1+0+22=23
We got the final result equal to 23. This means that the Easter holiday date for 2008 is March 23, 2008. (for the Gregorian calendar).
In the case of the Julian calendar, the calculations look identical but for A we substitute A=15 and for B we substitute B=6).
There is yet one method of calculating the date of Easter. It is the Meeus/Jones/Butcher method. It is simpler in that it does not require the numbers A and B, and there are no exceptions. The method proceeds as follows:
 First we divide the number of the year by 19 and determine the remainder a.
 Divide the number of years by 100, round the result down (cut off the fractional part) and we get the number b.
 We divide the number of years by 100 and get the remainder c.
 We calculate: b : 4 and the result is rounded down and we get the number d.
 We calculate: b : 4 and determine the remainder e.
 We calculate: (b + 8) : 25 and the result is rounded down and we get the number f.
 We calculate: (b  f + 1) : 3 and the result is rounded down and we get the number g.
 We calculate: (19 × a + b  d  g + 15) : 30 and determine the remainder h.
 We calculate: c : 4 and the result is rounded down and we get the number i.
 We calculate: c : 4 and determine the remainder k.
 We calculate: (32 + 2 × e + 2 × i  h  k) : 7 and we get the remainder l.
 We perform an action (a + 11 × h + 22 × l) : 451 and the result is rounded down and we get the number m.
 We perform the action: (h + l  7 × m + 114) : 31 and we get the remainder p.
 The result is x = p + 1 (it is Easter day)
 Month = Rounding down division (h + l  7 × m + 114) by 31.
For simplicity, there is a conversion between the Gregorian and Julian calendars. For the years 19002100, we simply add the number 13 to the dates on the Julian calendar and get the date for the Gregorian calendar.