What I found is that there is a (kind of) simple answer, and a more complicated one. This topic is a fun one for me, because it gets into one of my favorite areas: calendars. I am fascinated by the arbitrary and inaccurate ways in which we have decided to divide up our year. Have you ever wondered why oct-ober no-vember and dec-ember are the 10th 11th and 12th months instead of the 8th 9th and 10th months as their names suggest? That alone is reason enough to scrap our current system in my opinion. My mind was blown the first time I researched alternative calendars. My personal favorite is the International Fixed Calendar, which divides the months into perfect four-week intervals with the first always on a Sunday and the last always on a Saturday. Because each month contains the exact same number of work days, this is the calendar that the Kodak company used for years, and the one that I used when I was in sales (though no one else would cooperate with me).
Here goes with the simple answer:
Easter is scheduled each year on the first Sunday following the first full moon on or after the spring equinox. The equinox is Mar 21, and the full moon occurs the 14th day into the lunar cycle. Since the calendar has 12 months, but the moon makes its way around the earth 13 times in that same period, lining these two events up in this manner gives a wide range of available dates for Easter from as early as March 22 (as observed in 1818) to as late as April 25 (as it did in 1943).
But that's only if you happen to live somewhere that uses the Gregorian calendar, observe the fixed equinox, and go by the ecclesiastical lunar calendar. If not, you might be celebrating Easter in a completely different week.
Here is a summary of the long answer:
The very first Easter was the Sunday after Passover, a traditional Jewish holiday. Passover begins on the 14th or 15th day of Nisan (I got conflicting information), which is the seventh month of the Jewish lunar calendar. This date is called the vernal or spring equinox, or the Paschal moon. Passover ends 8 days later for traditional Jews, or 7 days later for reformed Jews. To further complicate things, Passover actually begins and ends at sundown, so there's that to deal with too. Here's a handy breakdown of when Passover falls in modern times.
The timing of Easter falling after Passover keeps the continuity of the gospels intact. The last supper was during Passover, and the following Sunday Jesus was resurrected. But it created an odd circumstance where the Jewish calendar was directly responsible for the placing of a Christian holiday on a calendar. That's not a big deal except that the 14th day of the Jewish lunar calendar should correspond with a full moon, but variances in the differences in the calendar and actual observed lunar events sometimes got so far off that at times Rabbis would just choose a liturgical full moon day and begin Passover then. This inconsistency made planning Easter tricky.
Rather than deal with that, some early church leaders decided to unhook Easter from Passover and make their own determination of when the first full moon was. They use an ecclesiastical fixed lunar calendar that disregards the observed lunar cycle, but makes calculating dates in advance quite easy. They assigned March 21 as the vernal equinox, and made Easter the Sunday after the first scheduled full moon on or after the equinox, whether the Jews agreed it was sufficiently full or not. Problem solved. Except only the West adopted this method, the East did not.
Another variance among Christians is the calendar used. While most countries adopted the Gregorian calendar between 1582 and 1927, certain orthodox religions still follow the Julian calendar. The difference is the Julian calendar was erroneous in its calculations of how often leap years should occur. When switching over to correct for this error, it meant eliminating 10-13 days from the year when the changeover occurred, depending on when they were adopting the new system. This created interesting situations along the way, like 18 day months for some. In one case, inexplicably, this resulted in a February 30th. If you did the conversion today, it would be 14 whole days. That's right, the Julian and Gregorian calendars are off by two whole weeks.
So even if they agree on when the Paschal moon occurs, the actual day Easter falls on may differ by as much as a couple weeks because of calendar discrepancies. This means that sometimes Eastern and Western dates for Easter can coincide, like they did on April 20, 2014. But the following year in 2015 the West celebrated Easter on April 5th, but the orthodox on the 12th. This also means that sometimes the West actually celebrates Easter before Passover. Crazy, huh?
Conclusion
When my sister and I were discussing this, I wanted to calculate the odds that any given day within the available Easter dates (like my nephew's birthday) would pop up. But even if we toss out the variances in how different cultures have decided to place the day, it still is rather complex to calculate the odds. You have 3 variables that must line up exactly for any particular date to be Easter: lunar calendar, regular calendar, and day of the week. I couldn't make it all the way through this Wikipedia article on the mathematics of choosing an Easter date, but maybe you can. There are tables and charts and algorithms galore, and I still have no idea what the odds are. I just know that twice in a lifetime was apparently a long shot.
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