Up until the Council of Nicea in 325 AD, the Church always celebrated Resurrection Day (Easter) on the Sunday after Passover. At Nicea the bishops changed the calendar, inventing the formula that says Easter will be celebrated on the "First Sunday after the first full moon after the vernal equinox." (Not exactly a biblical basis, is it?) The result, of course, is that Christians lost the connection between the holidays, which is most unfortunate. This is why we work so hard to do "Messiah in the Passover" presentations and seders, to help believers reconnect the holidays and see the fulfillment of the biblical type (the Passover lamb of Exodus) in Messiah Jesus (see 1 Cor. 5:7).