Passover traditionally begins in late March or early April — in 2017, Passover begins the evening of April 10 and ends the evening of April 18. The exact dates vary from year to year, because they’re based on the traditional Jewish calendar rather than the one we use today. The Jewish calendar is a lunar calendar — it’s organized around months set by the phases of the moon rather than years set by a full revolution around the sun. So while the first night of Passover is always the day on the Jewish calendar, it can vary on the calendar we use.
This is also why Easter, unlike other Christian holidays, “floats” around on the calendar: Easter almost always falls during Passover, because the New Testament says that this is when Jesus was crucified. (This is also why the Last Supper is often said to have been a Seder, and why TV networks always show The Ten Commandments on Easter.)
Originally, Jewish law called for Passover to last for seven days, with a Seder on the first night. However, as Jews spread around the globe, they became less certain that they’d be able to keep their calendar in line with the dates in Israel — so they expanded it to an eight-day holiday, with Seders on the first and second nights, just in case.
Now that many Jews do live in Israel, they tend to celebrate the original, seven-day Passover — and some American Jews have followed suit. This is one of many ways that the existence of Israel and of Israeli Jews has changed how Passover is understood and celebrated. For many Jews outside of Israel, it reminds them of the difference between being a people in exile (as Jews were for much of their history) and having a Jewish land that they choose not to live in — often sparking conversations around the table about how they feel about Israel and its actions today.