Easter, the greatest feast day in all Christendom, is determined astronomically by the longitude of the Sun and Moon in the Heavens, therefore it is not truly the anniversary of the resurrection of Jesus Christ. In 325 AD, the bishops of the Nicene Council of Rome decided to use the science of astronomy in order to establish uniformity in determining what day Easter would fall on. It was established that Easter would always be on the first Sunday after the first Full Moon after the Vernal Equinox.
Today (4/20/'03) is the first Sunday after the first Full Moon (4/16/'03) after the Vernal Equinox (3/21/'03).