Have you ever wondered why Easter dates change yearly while other festivals such as Christmas or St. Valentines have fixed dates? The variation is a result of an astronomical occurrence making Easter a movable celebration that can occur from March 22 to April 25. The Easter holiday coincides with the March Equinox hence the holidays do not have a fixed date in either the Georgian of Julian calendars, which are used by most Orthodox Christian Churches.Due to the variation of Easter dates, other celebrations are affected: Palm Sunday, Ascension Day, and Whitsunday/ Pentecost. Taking a closer look at the Bible reveals that the Jewish Passover was celebrated during the full moon after the Vernal Equinox. It was around the same time that Christ’s death and resurrection occurred. The different dates of the Vernal Equinox led Christians to celebrate Easter holiday on various dates. Early Christians wanted to preserve the order of observing Easter after the Passover. By the end of the 2nd century, most Christians celebrated Easter on the same day as the Passover, while others did it on the Sunday following the Passover. The first full moon during spring is called the Paschal full moon and was used to set different Easter dates. If the Paschal Full Moon is on a Sunday, the Easter holiday was to be pushed to the following Sunday.
In 325 
CE, the Council of Nicaea decided that Easter would be celebrated on the first Sunday after the occurrence of the Vernal Equinox. Since then Easter has been approximated to take place from March 21st. Despite the decision from the Council of Nicaea, some Christians do not follow the Georgian calendar and use the Julian calendar. The Georgian calendar was made because people thought that the Julian calendar was very long. The Julian calendar had its equinox at the early dates of March hence moving it farther away from Easter. The Georgian calendar corrected that and aligned the holiday with the equinox. From 1753-2400, the dates for Easter range from March 22 to May 10 according to the Julian and Georgian calendars. In some rare occasions, Easter dates are the same in both Julian and Georgian calendars. The occurrence happened in 2017 and could do so again in 2034.
You might wonder which Churches adopt which calendar. Eastern churches (they include Slavic and Greek) and Oriental Churches (the likes of Coptic Egyptian, Ethiopian, Syrian, and Armenian) use the Julian calendar, named after Emperor Julius Caesar. Most nations in Europe adopted the Georgian calendar in 1582, but Eastern and Oriental Churches still use the Julian calendar.
Recently, there have been heated discussions as to whether Easter holidays should be unified for both calendars. In 1997, the World Council of Churches proposed a solution that would entail the use of an Easter calculation that could replace the equation-based approach of determining Easter using astronomical observations. According to some of the leaders, it would help solve differences between churches that use the Julian and Georgian calendars. The proposed reform had a scheduled implementation in 2001, but it has yet to occur. In another attempt, the Archbishop of Canterbury, Justin Welby, admitted that he was holding talks with other Christian leaders to come up with a fixed date for the Easter holiday. However, with tension in the Syrian churches and problems in the Eastern churches, it might be difficult to achieve.