Resurrection Sunday, also known as Easter Sunday, is the celebration of Jesus Christ's resurrection from the dead, which is considered the cornerstone of the Christian faith. The timing of Easter Sunday is determined by the lunar calendar and falls on the first Sunday following the first full moon after the vernal equinox.
Churches have celebrated Easter Sunday since the earliest days of Christianity, as it represents the fulfillment of the promise of salvation and eternal life for believers in Jesus Christ. The practice of celebrating Easter Sunday can be traced back to the 2nd century, with the first recorded observance occurring in Rome in the year 155 AD.
Throughout history, the date of Easter Sunday has been a subject of controversy and disagreement among various Christian churches. In 325 AD, the Council of Nicaea established that Easter Sunday would be celebrated on the first Sunday following the first full moon after the vernal equinox. This formula, known as the "ecclesiastical lunar calendar," has been used by most Christian denominations ever since.
In summary, Easter Sunday is celebrated by churches to commemorate Jesus Christ's resurrection from the dead, which is considered the central event of the Christian faith. The date of Easter Sunday is determined by the ecclesiastical lunar calendar established by the Council of Nicaea in 325 AD.