On the Gregorian Calendar, which is now in common use for civil purposes around the world, it can come as early as March 22 and as late as April 25. That's a pretty wide spread!
It is, in fact, even wider than the spread for Chinese New Year, which can come as early as 21 January and as late as 20 February on the Gregorian Calendar. Chinese New Year is calculated according to the Lunar Calendar.
Generally speaking, Easter falls on the first Sunday that occurs following the first full moon that occurs on or following the Vernal Equinox (if you live in the Northern Hemisphere) or the Autumnal Equinox (if you live in the Southern Hemisphere).
On 21 March and 23 September, the suns shines directly on the equator. As a result, the length of day and night are approximately equal around the world. When this happens in the spring, it is called Vernal Equinox. When it happens in fall it is called Autumnal Equinox.
Known as a movable feast, Easter – and all of the holidays related to it, such as Palm Sunday, Good Friday, and Ash Wednesday – coincides with the Vernal Equinox. Why? According to the New Testament, Jesus was resurrected from the dead three days after his crucifixion.
This took place during the Jewish Passover, which was celebrated on the first full moon following the Vernal Equinox. Many early Christians starting celebrating Easter on the same day as Passover.
Others started celebrating it the following week. Dates varied from place to place and from church to church.
When in Rome
In 325 AD, Roman Emperor Constantine convened the Council of Nicaea in a bid to create uniform church doctrine throughout the world. One of the issues at hand was fixing a date for Easter. It was determined that it would take place on the first Sunday after the first full moon occurring on or after the Vernal Equinox.
Unless, that is, the full moon took place on a Sunday. In that case, Easter would be postponed until the following week. As a result of this determination, the chances of Easter and Passover coinciding were reduced.
Not all Christians celebrate Easter on the same date. Some Eastern Orthodox Christians continue to calculate Easter according to the Julian Calendar, which pre-dates the Gregorian Calendar.
Some years – such as this year (2010) – Easter coincides under both reckonings. Most years, however, it falls on a different date.
If all this sounds a bit complicated, it is! According to the US Navy, it takes more than 5 million years for the cycle of which dates Easter occurs on to run its full course.