Easter Sunday is the first Sunday following the full moon that occurs after the Spring Equinox, which is 21 March. This means that Easter Sunday can fall between 21 March and 28 April - it is all set out in advance, and has been defined this way for over a thousand years.
Easter Sunday is the first Sunday after the first full moon after the spring equinox, but never falling at the beginning of the Jewish passover. The Passover part is due to Constantine who was anti-Semetic. This rule was laid out at the council of Nicaea in 325.