In the early days of our Church, just as today, there were many different beliefs. The society was very agricultural, and many religions used the seasons to celebrate special feasts. It was important, in teaching about Jesus, to have people connect the Gospel message to their daily lives. Since Jesus' resurrection was the beginning of a whole new life and occurred so near the spring feasts, it was determined that Easter would always be calculated as the first Sunday after the first full moon after the vernal equinox (that is, the first day of spring).
originally appeared in Knowing Our Catholic Faith