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 astronomicalspring 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.