In 325CE the Council of Nicaea established that Easter would be held on the first Sunday after the first full moon occurring on or after the vernal equinox. From that point forward, the Easter date depended on the ecclesiastical approximation of March 21 for the vernal equinox.
The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar. Passover is a spring festival, so the 15th day of Nisan typically begins on the night of a full moon after the northern vernal equinox.
According to Chabad.org, the 2018 Passover is March 30-April 7.