Jesus rose from the dead on the first Sunday following the
feast of Passover. (Technically, he may have risen Saturday
night, but that still counts as Sunday on the Jewish reckoning,
which begins each day at sunset instead of at midnight.)
The date of Passover is a complicated thing. Theoretically, the
date should be the 14th of the Jewish month of Nisan, and it
should correspond to a full moon (the Jewish calendar being
partly lunar). In practice, it didn’t always work out that way.
The month-moon cycles got out of synch, and sometimes
feasts would be held on a "liturgical" full moon even when it
was not an astronomical full moon. As a result, rabbis
periodically had to announce when Passover would be
celebrated.
Christians didn’t like being dependent on the pronouncements
of rabbis for how to celebrate Christian feasts, so they came
up with another way of determining the date. They decided
that Easter would be celebrated on the first Sunday after
(never on) the Paschal full moon.
Theoretically, the Paschal full moon is the first full moon
occurring on or after the spring equinox. However, this day
can be reckoned in different ways. One way is by looking at
the sky, which yields the astronomical spring equinox. But
since this shifts from year to year, most people follow the
calendrical spring equinox, which is reckoned as March 21.
On the Gregorian calendar (the one that we use), Easter is the
first Sunday after the Paschal full moon, which is the first full
moon on or after March 21. Easter thus always falls between
March 22 and April 25.
Now, to find Palm Sunday (the sixth Sunday of Lent) you start
with the date of Easter and back up one week: It is the
Sunday before Easter Sunday.
To find Ash Wednesday, you start with the date of Easter
Sunday, back up six weeks (that gives you the first Sunday of
Lent), and then back up four more days: Ash Wednesday is
the Wednesday before the first Sunday of Lent.

Source: http://www.catholic.com/quickquestions/how-is-easter-sunday-determined-palm-sunday-ash-wednesday