Nisan (ניסן) is, traditionally, the first month of the Hebrew calendar. Sometimes 1 Nisan is called the New Year for Kings, but the major Hebrew new-year festival, Rosh Hashana, is kept on 1 Tishri.
Nisan always has thirty days and is a spring month (in the northern hemisphere). it corresponds with late March and early April in the Gregorian calendar.
The full moon in Nisan, 15 Nisan, is a major Jewish feast, Pesach or Passover. More recently, 27 Nisan has been kept as Holocaust Memorial Day, or Yom haShoah.