At a meeting of church leaders -called the Council of Nicaea- in 325 AD to try to coordinate practise it was decided that Easter was to be celebrated on the first Sunday after the full moon that directly followed the Vernal (or Spring) Equinox. In most years this would coincide with Passover, but it would not always be so. The Eastern Churches still retained the earlier Passover practice for several centuries, and the British (Celtic) Church did not adopt the Roman calendar until Roman missionaries arrived in the sixth century.

richmondparkchurch.org.uk an evangelical pentecostal church in Boscombe, Bournemouth, Dorset, UK