al-Nasīʾ (Intercalation)

Al-nasi': the people of the Jahiliyya moved forward and postponed the sacred months to suit the needs of war and trade, until the Hajj coincided with its true time in the tenth year, and then the practice was abrogated by the Qur'an.

The people of the Jahiliyya would move one month forward and postpone another. They would swap the status of one of the sacred months for another month when the need of war or business called for it. When they wished to perform the nasi', their people stood up at the Hajj season, making Muharram lawful and making Safar forbidden ¹.

This manipulation continued until the Hajj of the tenth year matched its true time. The Prophet then performed the Farewell Pilgrimage with the months in their original order ¹.

The Qur'an addresses this practice directly in al-Tawba 37 ¹.

Ibn Ishaq gives the interpretation of the verse on nasi' in Surat al-Tawba. The Arabs would make lawful one of the sacred months and forbid another in its place, to match the count of what God had forbidden. The verse was revealed condemning the practice, and it is not permitted to make lawful what God has forbidden or to forbid what He has made lawful ². See Abu Bakr leads the pilgrimage, 9 AH.