The intercalation doubled the month of the pilgrimage, that is, the month of the pilgrimage and the following month were given the same name, postponing the names and the sanctity of all subsequent months in the year by one. The first intercalation doubled the first month Muharram, then three years later the second month Safar was doubled, continuing until the intercalation had passed through all twelve months of the year and returned to Muharram, when it was repeated. The Arabs, according to one explanation mentioned by Abu Ma'shar, learned of this type of intercalation from the Hebrew calendar used by the Jews, since intercalation was announced by the Nasi, meaning "prince", or "ruler". The Hebrew calendar commanded by Moses son of Amram, Mūsā bin ʿImrān, in Exodus 12, is necessarily lunisolar, because the lunar new year is fixed to the month of Aviv, or spring, and cannot rotate through the year. Prohibition under Islam In the tenth year of the Hijra, according to chapter 9:36–37, a prohibition of Nasīʾ was enacted: The prohibition of Nasīʾ would presumably have been announced when the intercalated month had returned to its position just before the month of Nasīʾ began. If Nasīʾ meant intercalation, then the number and the position of the intercalary months between 1 AH and 10 AH are uncertain; Western calendar dates commonly cited for key events in early Islam such as the Hijra, the Battle of Badr, the Battle of Uhud and the Battle of the Trench, should be viewed with caution as they might be in error by one, two or even three lunar months. This prohibition was mentioned by Muhammad during the Farewell Sermon which was delivered on 9 Dhu al-Hijjah 10 AH (Julian date Friday March 6, 632) on Mount Arafat during the Farewell Pilgrimage to Mecca. The three successive forbidden months mentioned by Muhammad (months in which battles are forbidden) are Dhu al-Qi'dah, Dhu al-Hijjah, and Muharram, months 11, 12, and 1. The single forbidden month is Rajab, month 7. These months were considered forbidden both within the new Islamic calendar and within the pre-Islamic Meccan calendar. See also Islamic calendar References Islamic calendar Pre-Islamic Arabia Quranic words and phrases