In a leap year, Esfand has 30 days in the Solar Hijri calendar.
Esfand is the twelfth and final month of the Iranian Solar Hijri calendar, falling roughly across February and March in the Gregorian calendar. In an ordinary Solar Hijri year it contains 29 days; the extra day is added in a leap year, making 30.
The month-length pattern is distinctive. Farvardin through Shahrivar, the first six months, each have 31 days. Mehr through Bahman, the next five, each have 30 days. Esfand alone varies between 29 and 30 days, allowing the calendar year to stay aligned with the astronomical cycle.
Solar Hijri leap years are tied to the timing of the vernal equinox rather than simply following the Gregorian calendar's familiar four-year rule. The calendar begins around the March equinox, and Nowruz is placed according to the astronomical start of spring in Iran. A common mix-up is to assume Esfand always has 30 days, but that is true only in leap years.