So, we're celebrating Pascha this year before Passover has concluded; why?
Short answer: Because we follow a Paschalion
based on the Julian Calendar, not on actual astronomical events observable today.
Long answer: Our Paschalion
is not tied to the Jewish Passover. That's actually one of the two main points of the relevant canon passed at the First Ecumenical Council, which were:
(1) We don't follow the Jews. Not only is that something that a couple of heretical groups did, but, as Fr. George said, the Jews after the destruction of the Temple had lost their ability to properly calculate the Passover. In fact, it was so bad that they had to have their own council to address the issue a couple of generations after Nicaea. By the time of Zonaras and Balsamon, the major 12th century canonists you mentioned, the Jews had corrected things, so it appeared to those canonists that the calculations coincided, and thus they incorrectly claimed we depend on the Jewish Passover. Such was just coincidence.
(2) The Patriarch of Alexandria (having at his disposal the best scientists and astronomers) should send a letter to all the churches each year, informing them of the date of Pascha. He should use those scientists to determine the actual date of Pascha, which has two characteristics: (1) It's on a Sunday, (2) following the full moon of the spring equinox.
So, the Patriarch of Alexandria went home, went to the best scientists of the day, and they (rightly) determined that the spring equinox was occurring on March 21 at that time. They therefore drew up a Paschal calendar (Paschalion
) which stated that the earliest possible day for the celebration of Pascha is March 22, if the full moon occurs on March 21, and the latest is April 25, if the full moon occurs on April 18.
The only problem? The Patriarch of Alexandria didn't continue to calculate an up-to-date Paschalion
, based on when the spring equinox actually
happens. Instead, we more or less follow the original calculation, pretending that the equinox is occurring on March 21, as calculated on the Julian calendar. As the Julian calendar has gotten less and less accurate, so too has our calculation of the full moon following the spring equinox.