Now the two dates for Christmas are easily explained: most of the Eastern churches (not all) use the older Julian calendar as their liturgical calendar, the West uses the Gregorian. The East celebrates Christmas on January 7 because, at the moment, there are 14 days' difference between the two calendars. It used to be January 6 (sometimes called "Old Christmas" in English speaking countries, which only made the change in 1752, whereas Catholic countries changed in 1582) As time goes on, it will shift to January 8, as the two calendars drift farther apart. That's not all that complicated.
Easter, on the other hand, is a whole different matter. The Julian/Gregorian calendar difference is part of it today, but the two halves of Christianity set up different mechanisms for calculating the date for Easter long before the calendar reform. Their differences were multiple, ranging from how they calculated the lunar cycle to how long a cycle it took for dates to repeat themselves; this sas further complicated with the introduction of the Gregorian calendar.
I was going to explain it thoroughly, but then I encountered passages like this, from Wikipedia, and figured, the hell with it:
In determining the date of the Gregorian and Julian Easter a lunisolar cycle is followed. In determining the date of the Jewish Passover a lunisolar calendar is also used, and because Easter always falls on a Sunday it usually falls up to a week after the first day of Passover (Nisan 15 in the Hebrew calendar). However, the differences in the rules between the Hebrew and Gregorian cycles results in Passover falling about a month after Easter in three years of the 19-year cycle. These occur in years 3, 11, and 14 of the Gregorian 19-year cycle (corresponding respectively to years 19, 8, and 11 of the Jewish 19-year cycle).
The reason for the difference is the different scheduling of embolismic months in the two cycles.
In addition, without changes to either calendar, the frequency of monthly divergence between the two festivals will increase over time as a result of the differences in the implicit solar years: the implicit mean solar year of the Hebrew calendar is 365.2468 days while that of the Gregorian calendar is 365.2425 days. In years 2200–2299, for example, the start of Passover will be about a month later than Gregorian Easter in four years out of nineteen.
Since in the modern Hebrew calendar Nisan 15 can never fall on Monday, Wednesday, or Friday, the seder of Nisan 15 never falls on the night of Maundy Thursday. The second seder, observed in some Jewish communities on the second night of Passover can, however, occur on Thursday night.
Because the Julian calendar's implicit solar year has drifted further over the centuries than those of the Gregorian or Hebrew calendars, Julian Easter is a lunation later than Gregorian Easter in five years out of nineteen, namely years 3, 8, 11, 14, and 19 of the Christian cycle. This means that it is a lunation later than Jewish Passover in two years out of nineteen, years 8 and 19 of the Christian cycle. Furthermore, because the Julian calendar's lunar age is now about four to five days behind the mean lunations, Julian Easter always follows the start of Passover. This cumulative effect of the errors in the Julian calendar's solar year and lunar age has led to the often-repeated, but false, belief that the Julian cycle includes an explicit rule requiring Easter always to follow Jewish Passover. The supposed "after Passover" rule is called the Zonaras proviso, after Joannes Zonaras, the Byzantine canon lawyer who may have been the first to formulate it.Ah, yes, The Zonaras proviso. So, even though the Eastern date of Easter always falls after Passover, there's no rule that says it has to; it's just the "cumulative effect of errors"? But wait, then what is the Zonaras proviso, if not a rule?
Anyway, the short answer is that the sun, the moon, the calendar, and the date of Passover all factor in, and the Eastern and Western formulas can give the same date, but more often don't. Since this year the Western Easter coincides with Passover almost exactly, the Eastern date has to be later, but only by a week.
Clear? Then could someone explain it to me?
Anyway, a happy Easter to anyone celebrating it, on either date. My Passover post will be along later today sometime.