The Easter Calendar, one of the rarest and most significant pieces of the Archiepiscopal Museum in Ravenna, can be seen on the east wall. The calendar was produced by engraving a square slab of marble quarried on the island of Proconnesus located in the Sea of Marmara. The calendar, which is a unique piece, was made for the purpose of determining the date on which to celebrate Easter from AD 532 to AD 626.
As we all know, Easter falls on a different day each year depending on the phases of the moon. The Council of Nicaea, which took place in AD 325 determined that the Christian Easter should never fall on the same day as the Hebrew Easter. The Hebrew Easter was celebrated on the fourteenth day after the full moon of March.
The Council of Nicaea therefore established the date of the spring equinox as March 21st and fixed the date of the Christian Easter as the first Sunday after the first full moon after the spring equinox.
The calendar of Ravenna gives a graphic representation of the rule of Dionysius Exiguus (Dionysius the Humble), in other words the tables for computing the date of Easter, adopted by the Church in the west starting from AD 532 because this was the number of years in an entire cycle in which the date of Easter changes.