It is true that Passover and the Feast of Unleveaned Bread, together, are known to the Jews as Pesach, or its English equivalent as Passover Festival. It is held from the 14th day to the 21st day of the first month of the religious calendar.
During the time of Jesus, the Jews were using two kinds of calendars: a purely lunar religious calendar like the Muslim calendar, and a lunisolar civil calendar. The purely lunar calendar has no intercalation so it is wandering through the seasons. In the lunisolar calendar, a month is intercalated on the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth year in a 19-year Metonic cycle, to keep the calendar in season. In 358/359 AD, under Hillel II, the Jews reformed their calendars and came out with just one calendar, from two calendars, which we know today.