Maggie raised the question as to when we Orthodoxen will celebrate Easter.
Basically, the Orthodox Church follows the decision of the Council of Nicea in 325, which stated that Easter (Pascha) is the first Sunday after the first full moon after the vernal equinox, which marks the start of spring and astronomically is fixed as March 21st.
If the first full moon falls on a Sunday, then Eater/Pascha is transferred to the next available Sunday.
So this year, Passover starts at sundown on Sat 19th April. The full moon is on Sunday the 20th, so Pascha is transferred to the next Sunday, which is the 27th April.
The original Jewish tradition in force at the time of Jesus was that Passover was celebrated on the first full moon after the vernal equinox, so there was normally a tie-in between the dates of Easter and Passover. Once the Temple was destroyed and the Diaspora of the Jewish people began, the date of celebrating Passover gradually shifted, so that on occasion, it can be celebrated before the vernal equinox.
The Calendar issue also muddies the waters, as the Orthodox church continues to calculate the date of Easter according to the Julian calendar in use at the time of Nicea, and which was abandoned by the west several hundred years ago .....so that skews things by 13 days anyway.
For the Orthodox, Pascha would not normally be celebrated before March 21st on the Julian calendar, which is April 3rd on the Gregorian Calendar in use by the West.
Just to add to the complexity, there continued to be technical difficulties in accurately reckoning the appropriate date even at the time of Nicea, so the concept of Paschal cycles was developed to take into account variations in astronomical cycles/issues, and in the East these cycles were 19 years in length. In the west, these cycles lengthened to 84 years.
So that is how we get the variations in celebrating Easter, East and West. The cycles can and do coincide; In 2010, East and West will celebrate together on April 4th, and in 2011 upon April 24th.
In 2013, the East will celebrate Pascha on May 5th, which is really late :-)