Pesach (pronounced PAY-sakh) is the Hebrew word for Passover. Pesach is a holiday that commemorates the Hebrews??? rapid departure from ancient Egypt.
Passover is a Spring holiday; it starts on the 15th of Nissan (usually sometime in April) and lasts for eight days (in Israel, seven days). The first two and last two days (in Israel, only the first and last day) are major holidays, i.e. on these days it is forbidden to work, drive, turn on or off a light, etc. The middle days are Chol Hamoed.
Who knows how is the date for Passover determined?
The 15th of Nissan is the date of the Exodus. Now if you are asking how that translates in to dates on the secular calender, that's a bit more complicated. You are try to bring lunar and solar calenders into juxtaposition. Unfotunatly there is no somple formula. Froma practicle point of view the easiest thing to do is to use a Jewish calender and there you are.