A Gregorian and Julian Easter, how do they differ?

Christians around the world this April will be celebrating their holiest holiday, the death and resurrection of Jesus Christ. For some, who adhere to the Gregorian calendar, Good Friday will be on Friday the 19th, while those who follow the Julian calendar will be waiting until Friday the 25th. Both celebrate the life and death of a central Christian figure, so how and why do they differ? And how are the dates of Easter decided?

Hand decorated Easter eggs are a tradition in many European countries. Source: SBS