Easter: why does the date vary from year to year?

If we are not sure of the year of the death of Jesus of Nazareth, we are sure that it was a 14 nisan, nisan being the month of hebrew calendar beginning at the new moon falling on the day of theequinox of spring, or immediately after, that is to say the new moon following March 21 of our calendar. The date of the Christian Passover is fixed on the first Sunday after the full moon following thespring equinox.

It therefore depends on the Sun (for the equinox) and the Moon, which explains why it cannot be fixed in our solar calendar or in a lunar calendar. It can only be in a lunisolar calendar as is the Hebrew calendar. Thus, in 2018, Easter Sunday will be the 1^er april. In 2019 it will be April 21 and in 2020 it will be April 12.

And Easter in the East?

Added to these new moon counting complications is the use of the old Julian calendar in the East. March 21 in this calendar coincides with April 3 Gregorian. Thus, in 2018, the Orthodox Easter Sunday will be Gregorian April 21, which corresponds to Julian April 8. In 2019 it will be April 28 and in 2020 it will be April 19. The rule is actually simple. If a full moon occurs between March 21 and April 3, the dates are separate. Otherwise, they are identical. Thus, they will be identical in 2025, 2028 and 2034, distinct in the other years.