Catholics and Protestants will celebrate Easter this weekend while Orthodox Christians will celebrate Pascha a week later. Why the difference?
The first difference is the term for the Feast itself: following the practice of adopting modified pagan names for seasons and days of the week (i.e. Thursday is from the Norse god meaning 'Thor's day') Easter is derived from the pagan goddess of spring 'Eostre' and April was referred to by the Venerable Bede as the 'Month of Eostre'. Pascha is taken from Passover, the Jewish feast on which the true Pascal Lamb, Jesus Christ, was offered for the sins of the world.
Initially, Christians celebrated Pascha near the Jewish Passover but as time passed the Church adopted the formula that Pascha would be the first Sunday after the first full moon after the vernal/spring equinox after the Jewish Passover.
The universally accepted dating of Pascha was followed until Catholic Pope Gregory XIII updated the calendar in 1582. The Catholics set the fixed date of March 21 as the equinox and disregarded the date of the Jewish Passover. The Orthodox maintained the astronomical equinox (in Jerusalem) and the provision that Pascha must never occur before the Jewish Passover (sometimes Western Easter is before the Jewish Passover). This was also the chief factor in the general rejection of the Gregorian Calendar initially by the Orthodox Churches and Orthodox nations (Russian did not use the Gregorian calendar until 1918).
This year the celebrations are just a week off...next year Western Easter is March 27 while Pascha is May 1!