Then the Nicene Synod intervened in A.D. 325 and commanded all the Eastern Churches "which heretofore used to celebrate the Pasch with the Jews,"[196] to celebrate it in future at the same time with the Romans and with us--so say the prelates of the Synod in their circular letter to the Egyptian Churches. Constantine, the Emperor, in his own circular says that the Synod agrees that all should celebrate the Pasch on the same day, but that it should never be on the same day with the Jews; and Cyril of Alexandria states, and Leo the Great confirms the statement, that the Alexandrian Church was to calculate the dates, and then notify them to the Roman Church, which was to convey the information to the other Churches. This was virtually adopting the Alexandrian cycle of nineteen years--which was very different from the Roman cycle. Then at Alexandria the equinox was rightly fixed on the 21st March, at Rome it was the 18th; at Alexandria they celebrated Easter on the 15th day of the moon, _when the 14th was a Saturday_; at Rome they did not celebrate Easter in any circumstances before the 16th day of the moon--assuming that as the 14th day represented Good Friday, the Pasch of the Passion, Easter Sunday, the Pasch of the Resurrection, could not rightly take place before the 16th. It is curious that Cummian in his Epistle supports this opinion, although Bede makes the 15th of the moon a possible Easter Sunday, and such is still the usage. A diversity of practice, therefore, between Rome and Alexandria still continued for many years. However, the Alexandrian usage ultimately prevailed, but was finally accepted in the Western World only about A.D. 530, when explained and developed by Dionysius Exiguus. This, the correct system, therefore, lays down three principles. First, Easter Day must be always a Sunday, never on, but _next after_ the 14th day of the moon. Secondly, that 14th day, or the full moon, should be that on or next after the vernal equinox; and thirdly, the equinox itself was invariably assigned to the 21st of March.