INTERCALATION of the Year.
Arabic nasi. The privilege of commuting the last of the three continuous sacred months for the one succeeding it, the month Safar in which case Muharram became secular, and Safar sacred. M Caussin de Perceval supposes that this innovation was introduced by Qusaiy, an ancestor sixth in ascent from Muhammad, who lived in the middle of the fifth century. Dr. Sprenger thinks that intercalation in the ordinary sense of the word was not practiced at Makkah, and that the Arab year was a purely lunar one, performing its cycle regularly, and losing one year in every thirty-three.
The custom of nasi’ was abolished by Muhammad, at the farewell pilgrimage, A.H. 10, as is stated in the Qur’an, Surah ix. 36, 37: –
“Twelve months is the number of months with God, according to God’s book, since the day when He created the heavens and the earth: of these four are sacred; this is the right usage. But wrong no yourselves therein; attack those who join gods with God in all, as they attack you in all: and know that God is with those who fear Him.”
“To carry over a sacred month to another, is only a growth of infidelity. The Infidels are led into error by it. They allow it one year, and forbid it another, that they may make good and number of months which God hath hallowed, and they allow that which God hath prohibited.”
Based on Hughes, Dictionary of Islam