Today, 16 April all Christians celebrate the Easter. This is a rare occasion as two largest churches – catholic and orthodox, calculate their holiday dates differently and as a result more often that not the Easter dates of these churches are different. The most prominent reason for this is the fact that these churches use different calendars. Catholic church uses the Gregorian calendar, whereas the Orthodox church uses the old Julian calendar. This is also the reason that the dates of Christmas are different too. Unlike Christmas however, Easter date is not static and is calculated on a yearly basis and according to different rules. The Orthodox church requires the Easter date not to correspond with Jewish Passover. This is a rule set back in 325 by First Ecumenical Council in Nicea. As for the Easter date calculation, the council decided for Easter to be held on first Sunday after the first full moon after March 21 (also known as the vernal equinox – the day when on equator the day and night are nearly the same lengths) The Catholics only partially adhere to this rule, ignoring the date of the Jewish Passover. Theoretically, if the first moon happens on or before Jewish Passover on Sunday, the Orthodox church will postpone the date resulting in a huge gap compared to Catholics.