The Christians of the 2nd Century, the date most accepted by religious historians as the beginnings of Easter celebration by Christians, used different calenders depending on their geography. The Julian Calender was the most widely used up until the 16th century, but the Julian Calender was off by 11 minutes and 14 seconds every year. Pope Gregory XIII commissioned a study to find a way to correct it and stop it from drifting into the future. This lead to leap years. The new calender was called the Gregorian calender. However, the Orthodox Church still uses the Julian calender...so different Christian groups celebrate Easter on different days.