Justifications in the Iranian Official Calendar Definition
A major justification in the Iranian Official Calendar was changing number of days in each month in order to improve yearly calculations. According to these modifications, the calendar consists of 12 months, the first six of which are 31 days, the next five 30 days, and the final month 29 days in a normal year and 30 days in a leap year. Whereas, in ancient Persian Calendar: each of the 12 months had 30 days. This Avestan Calendar of 360 days required regular correction to keep it synchronized with the solar year; this was achieved by intercalating a 13th month roughly once every six years.Those changes sometimes create confusion in determining the exact date of Iranian Festivities.