According to the four Canonical gospels Jesus Christ and the apostles kept the Passover (John records three Passovers celebrated by Jesus: John 2:13, John 6:4, John 11:55 ); Why Christians today celebrate a different feast that appeared c. 150 AD, almost one century and a half later?
Christians celebrate Easter because it marks the anniversary of Jesus' Resurrection, the sole reason Christianity even exists and the source of the joy of salvation for the Christian. It is the most important event in Christianity and, according to Christians, all of human history.
Passover is the oldest Jewish celebration. It celebrates the Exodus in general and specifically how the Lord's Angel of Death passed over the households with the blood of a lamb on its door posts, but proceeded into the households of the Egyptian and took the first born. Passover is also a sacrificial event. A lamb is slaughtered for every Jewish family, as God commanded.
Christian theology is rich with analogy about how the Passover represents Christ's death, which covers our sins and saves us from death. Easter is the completion of that theology, where even Christ, the perfect sacrifice, conquers death and rises to Glory.
Many Christians are not Jews. Every Christian I've heard of does not believe that sacrifice is necessary anymore. Christ was the final sacrifice. It seems odd that Christians would celebrate an event that has been superseded by another: Good Friday. Good Friday is the celebration of Christ's Passion (march to the Cross, in perfect submission to the Father). Tradition holds that this took place on Passover.