Both al-Biruni and al-Mas'udi suggest that the Ancient Arabs used the same month names as the Muslims, though they also record other month names used by the pre-Islamic Arabs.The Islamic tradition is unanimous in stating that Arabs of Tihamah, Hejaz, and Najd distinguished between two types of months, permitted (ḥalāl) and forbidden (ḥarām) months.Another account relates that they used to loot the houses of their enemies after defeating them in battle, leaving nothing behind.This is the second sacred month in which fighting is forbidden.This interpretation was first proposed by the medieval Muslim astrologer and astronomer Abu Ma'shar al-Balkhi, and later by al-Biruni, This interpretation considers Nasī’ to be a synonym to the Arabic word for "intercalation" (kabīsa).
The Qur'an links the four forbidden months with Nasī’, a word that literally means "postponement".
maintain that the pre-Islamic calendar used in Central Arabia was a purely lunar calendar similar to the modern Islamic calendar.
According to this view, Nasī’ is related to the pre-Islamic practices of the Meccan Arabs, where they would alter the distribution of the forbidden months within a given year without implying a calendar manipulation.
The forbidden months were four months during which fighting is forbidden, listed as Rajab and the three months around the pilgrimage season, Dhu al-Qa‘dah, Dhu al-Hijjah, and Muharram.
Information about the forbidden months is also found in the writings of Procopius, where he describes an armistice with the Eastern Arabs of the Lakhmid al-Mundhir which happened in the summer of 541 AD.
It is also used by Muslims to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca.