I would just like to add what already has been said. The Orthodox calculate Pascha based upon the Alexandrian tradition approved at the Council of Nicea. We use a 19 year cycle which is commonly known as the Metonic cycle, also refered to as the Callipic Cycle (you can google this to find out what its all about). The Jews use the same 19 year cycle and a similar calendar as we do, when calculating their Passover approved in 363 a.d.
Thus Pascha always falls on the first Sunday AFTER the first full moon following the vernal equinox. Jewish Passover of course begins on the firstfull moon, thus if the full moon falls on the weekend, Orthodox Pascha is moved up one week, since the Nicene stipulation is that its the first Sunday AFTER the full moon.