Two major factors were involved here. (1) There was no exact "day" (other than Sunday) insisted upon for the resurrection in Christian tradition, only an approximate time; (2) Christians at some point took a non-Christian observance (Spring festival) and attempted to Christianize it (Resurrection observance) since the two occurred fairly close on the calendar. But since Easter was tied the Spring festival which is a seasonal event not a particular date (like a birthday), its timing is based on the sun which changes slightly from year to year. In this case, it's the vernal equinox which occurs typically in late March. Now because Christians felt it was important to distinguish the Sunday-significance from traditional Judaism (Saturday worshipping), Christians felt that honoring the resurrection on a Sunday was more important than pinpointing a consistent day-of-the-month on the calendar (unlike Christmas). So Easter falls on the first-Sunday on or after the vernal equinox. When the equinox falls late and after a Sunday, Easter is pushed off until the next Sunday which may be in April. Essentially, it's a pragmatic compromise rooted in a decision and tradition made far beyond our time.

Easter falls on the first Sunday after the first full moon after the spring equinox. The equinox can fall on March 20 or 21 but the rules for Easter specify March 21 as the date for the calculation. So Easter could be as early as March 22 or as late as April 25.

While the other answers are pretty accurate, there is one detail missing: Why Sunday? Sunday was chosen because Christ was crucified, according to scripture, the day before the Jewish Sabbath (Saturday) and was in the tomb for the better part of 2 days, returning to life the morning of the third, which would have been Sunday. According to Jewish custom, criminals who were to be executed had to be done before the Sabbath and buried accordingly, since the Sabbath is supposed to be a day of rest. Additionally, dealing with dead bodies was a complicated process, criminal or not, so the body had to be prepared and interred prior to the commencement of the Sabbath, which begins at sundown on Friday. Thus, Easter Day is always on Sunday, as is the Christian Sabbath.

Because nothing can be simple anymore.

You are all incorrect. It has nothing at all do do with any equinox. Easter Sunday is the Sunday following the Paschal Full Moon date for the year.

because, the church has their own calendar!!

It depends on when the cocoa is harvested in Venezuela....

The date for Easter Sunday was set by the Church Council of Nicea in 325 AD. It is set in the Western calendar based in part on when Passover falls. The holiday falls on the first Sunday after the first paschal (Passover) full moon after the vernal (spring) equinox in the northern hemisphere. It cannot fall earlier than March 21st or later than April 25th. Because the cycle of the moon is not consistent year to year (because of the interaction of the Earth's rotation with the rotation of the moon around the Earth) the date changes year to year. The Orthodox Church observes the same rule, but because it uses a different calendar, the dates between the Western and Eastern churches can vary by as much as a month, with Orthodox Easter never falling earlier than April 1 or later than May 7.

4-12-2017 Easter is determined by the lunar calendar, and nobody is sure how that works. Here is an attempt to explain it: http://www.viewzone.com/calendarx.html Christians make a big deal about Sunday because about two thousand years ago there was a big movement to avoid any resemblance to Jews, and the Christian church was taken over by the descendant of the Babylon Mystery Religion. There are lots of books with that title explaining the history.