Date of Easter main subject date of Easter
GN = 1 + (Y mod 19)That is, the year number Y in the Christian era is divided by 19, and the remainder plus 1 is the golden number. (Some sources specify that you add 1 before taking the remainder; in that case, you need to treat a result of 0 as golden number 19. In the formula above we take the remainder first and then add 1, so no such adjustment is necessary.) Cycles of 19 years are not all the same length, because they may have either four or five leap years. But a period of four cycles, 76 years, has a length of 76 × 365 + 19 = 27,759 days (if it does not cross a century division). There are 235 × 4 = 940 lunar months in this period, so the average length is 27759 / 940 or about 29.530851 days. There are 76 × 6 = 456 usual nominal 30-day lunar months and the same number of usual nominal 29-day months, but with 19 of these lengthened by a day on leap days, plus 24 intercalated months of 30 days and four intercalated months of 29 days. Since this is longer than the true length of a synodic month, about 29.53059 days, the calculated Paschal full moon gets later and later compared to the astronomical full moon, unless a correction is made as in the Gregorian system (see below).
The paschal or Easter-month is the first one in the year to have its fourteenth day (its formal full moon) on or after 21 March. Easter is the Sunday after its 14th day (or, saying the same thing, the Sunday within its third week). The paschal lunar month always begins on a date in the 29-day period from 8 March to 5 April inclusive. Its fourteenth day, therefore, always falls on a date between 21 March and 18 April inclusive, and the following Sunday then necessarily falls on a date in the range 22 March to 25 April inclusive. 
This is true of both the Western system (in the Gregorian calendar) and of the Eastern system (in the Julian calendar). In the solar calendar Easter is called a moveable feast since its date varies within a 35-day range. But in the lunar calendar, Easter is always the third Sunday in the paschal lunar month, and is no more "moveable" than any holiday that is fixed to a particular day of the week and week within a month, such as Thanksgiving.If one does ask the question of what the distribution would be in the long term, that is, over the whole 5.7-million-year period after which the dates repeat, this distribution can be found fairly simply, and is quite different from the distribution in the period 1900 to 2199, or even the distribution over the period since the reform until now. The date of Easter in a given year depends only on the epact for the year, its golden number, and its dominical letter, which tells us which days are Sundays (more precisely, the dominical letter for the part of the year after February, which is different in leap years form the letter for January and February). (The golden number only matters when the epact is 25, as explained earlier.) If we go forward 3,230,000 years from a particular year, we find a year at the same point in the 400-year Gregorian cycle and with the same golden number, but with the epact augmented by 1. Therefore, in the long term, all thirty epacts are equally likely. On the other hand, the dominical leters do not all have the same frequency  – years with the letters A and C (at the end of the year) occur 14% of the time each, E and F occur 14.25% of the time, and B, D, and G occur 14.5% of the time. Taking into consideration the complication having to do with epact 25, this gives the distribution shown in the second graph. April 19 is the most common because when the epact is 25 the ecclesiastical full moon falls on April 17 or 18 (depending on the golden number), and it also falls on these dates when the epact is 26 or 24, respectively. There are seven days on which the full moon can fall, including April 17 and April 18, in order for Easter to be on April 19. As a consequence, 19 April is the date on which Easter falls most frequently in the Gregorian calendar, in about 3.87% of the years. 22 March is the least frequent, with 0.48%.The relation between lunar and solar calendar dates is made independent of the leap day scheme for the solar year. Basically the Gregorian calendar still uses the Julian calendar with a leap day every four years, so a Metonic cycle of 19 years has 6,940 or 6,939 days with five or four leap days. Now the lunar cycle counts only 19 × 354 + 19 × 11 = 6,935 days. By not labeling and counting the leap day with an epact number, but having the next new moon fall on the same calendar date as without the leap day, the current lunation gets extended by a day, and the 235 lunations cover as many days as the 19 years (so long as the 19 years do not include a "solar correction" as in 1900). So the burden of synchronizing the calendar with the moon (intermediate-term accuracy) is shifted to the solar calendar, which may use any suitable intercalation scheme, all under the assumption that 19 solar years = 235 lunations (creating a long-term inaccuracy if not corrected by a "lunar correction"). A consequence is that the reckoned age of the moon may be off by a day, and also that the lunations that contain the leap day may be 31 days long, which would never happen if the real moon were followed (short-term inaccuracies). This is the price of a regular fit to the solar calendar.
From the perspective of those who might wish to use the Gregorian Easter cycle as a calendar for the entire year, there are some flaws in the Gregorian lunar calendar (although they have no effect on the paschal month and the date of Easter):