The proposal comes within joint efforts by Pope Tawadros and Pope Francis to work towards unifying the Church. Pope Tawadros’s proposal suggested the second Sunday in April to celebrate Easter.
The date of Easter has not been constant throughout the years. Since the Nicene Council in AD325 it has been moving according to an annual four-to-five-week cycle that has to do with the first full moon on or after the vernal equinox.
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http://en.wataninet.com/coptic-affairs-coptic-affairs/church-affairs/popes-francis-and-tawadros-agree-one-easter-for-all/14914/