The dates of Passover are determined by the Hebrew calendar, which functions differently than the Gregorian calendar. Passover occurs on specific dates, the 15th day through the 21st day, during the month of Nisan, which is part of the 12 standard months that make up the Hebrew calendar.
Though Passover happens during specific days in a specific month, the Hebrew calendar sometimes incorporates an additional month to even out the days of the year. This helps explain why the dates of Passover are variable from year to year according to the Gregorian calendar. Passover typically occurs in springtime, within a few weeks of the Christian holiday of Easter.