It is a solar calendar with 12 months and either 365 or 366 days. The new year starts when the Sun crosses the vernal (spring) equinox, as observed in Tehran. If this is before noon then that is New Year's Day, if after noon then the following day is New Year's Day. In theory this is judged by observation but in practice astronomical mathematicians predict the moment of coincidence each year. The year usually starts on 21st or 22nd of March (Gregorian), depending on the relationship with the Gregorian calendar at that time.
The months in each year are named as follows. Farvardin, Ordibehesht, Khordad, Tir, Mordad and Shahrivar each having 31 days; Mehr, Aban, Azar, Dey and Bahman each having 30 days and Esfand having 29 days or 30 in a leap year.