Easter is celebrated on the primary Sunday that follows the day of full moon which occurs first when the spring has started. It is observed that Easter as per the above mentioned calculation can come some day between 22nd of March and 25th of April. It is also seen that the Easter that is celebrated by the churches that are orthodox in the Eastern side is different from the one celebrated by the western churches.
The calculations are different based on some reasons. One of the reasons is that some churches decided to consider Easter in relation with Passover followed by Jews. The death of Jesus, His burial and rejection took place after the Passover. So, the Fathers in the church wanted the Easter to be celebrated just after the Passover. As the calendar for the Jews is dependent on solar cycle and lunar cycle, the day of celebration of the festival will change from one year to next year.
The Sunday that comes after the full moon that is named as Paschal which is observed just after the spring equinox, is also said to be calculated as Easter prior to 325AD. The day of full moon that is named as Paschal is normally calculated based on the historical events and not lunar events. The astronomers have already evaluated the dates when the full moon days will fall in the years to come. These calculations are utilized by the churches in the west to prepare a table which consists of dates of full moon associated with Ecclesiastical events. These days are considered as sacred days in the calendar associated with Ecclesiastical events.
This calendar was first formed in the year 1583AD. From then onwards, the full moon days were established well and were used for calculating the dates for Easter celebration. As per the Ecclesiastical calculations in the prepared tables, full moon named as Paschal is observed as the first full moon date after March 20. March 20th is seen as the day of vernal equinox in 325 A.D. Therefore, western churches celebrate Easter on the initial Sunday that comes following the full moon day named as paschal. But the full moon which is named as paschal can differ from the usual full moon by about one day or two days. The full moon day named as Paschal can occur between March, 21st and April, 18th. Hence, Easter can occur between 22nd of March and 25th of April.