Innama annasee-o ziyadatun fee alkufri yudallu bihi allatheena kafaroo yuhilloonahu AAaman wayuharrimoonahu (surah Tawbah: 37)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(9:37) Nasi is but one more instance of disbelief, whereby these disbelievers are led astray. In one year they make a certain month lawful and in another year they make the same a forbidden month, so as they might make up for the number of the forbidden months and, at the same time, make lawful that which is forbidden by Allah. *37 Their evil deeds have been made seeming fair to them: for Allah does not guide those who deny the Truth.

Innamma naseee`u ziyaadatun filkufri yudallu bihillazeena kafaroo yuhil loonahoo `aamanw wa yuhar rimoonahoo `aamalliyu watti`oo `iddata maa harramal laah; zuyyina lahum sooo`u a`maalihim; wallaahu laa yahdil qawmal kaafireen

*37) The pagan Arabs practised s. (nasi) in two ways. Whenever it suited them, they would declare a prohibited month to be an ordinary month in which fighting, robbery and murder in retaliation were lawful for them. Then they would declare an ordinary month to be a prohibited month instead of this month in order to make up for the deficiency caused in the number of the prohibited months.
The other way of nasi was the addition of a month in order to harmonize the lunar with the solar year so that the Haj should always fall in the same season and they should be saved from the botheration and inconveniences that are experienced by its observance according to the lunar year. In this way Haj was observed for 36 years on dates other than the actual dates. Then in the thirty seventh year it was again observed on the 9th and 10th of Zil-Hajjah, as they should have actually fallen according to the lunar calendar. It so happened that on the occasion when the Holy Prophet performed Hajja-tul-Wida'a (his Last Haj). these dates by rotation coincided with the actual dates of Haj according to the lunar calendar. So he declared during the course of his historic address at `Arafat: "This year the dates of Haj have again occurred exactly at their natural and proper time after a long period of circulation. " Accordingly, since that `Last Haj' of A.H. 9, it is being observed up to this day according to the correct dates of the lunar year.
By making nasi unlawful and forbidding it, the Law declared that the two objects for which it was practised were also sinful. First, they not only made lawful what Allah had made unlawful but also tried to practise deception on AIIah by the seeming observance of the Law. As regards the second object of making the season of Haj fixed permanently according to the solar calendar, this too, was in fact the worst kind of rebellion against the Divine Law, although it appeared to be a harmless thing done for public good. But it defeated the very object of the Divine Law which prescribed the lunar calendar instead of the solar calendar for the observance of obligatory duties. Allah did this to ensure many good things for the Muslims, one of which is that His servants should be so trained as to obey His Commandments at all times of the year and under all circumstances. For instance, the month of Ramadan circulates among all the seasons of the year and the Muslims get used to obeying His Commandments whether it be summer or winter, dry or rainy season. This gives them that excellent moral training which is the real object of all the obligatory duties imposed by the Divine Law. Likewise, the dates of Haj according to the lunar calendar, fall during different seasons: therefore the servants of Allah have to travel during all the seasons, favourable and unfavourable. Thus they come out successful in the Divine test under alI the circumstances and learn to be steadfast in their obedience.
It is obvious that such a training will stand them in good stead in many other fields in which one has to fight the battle of the Truth.
Now it would have become quite clear why nasi was made unlawful and forbidden. It was designed to fix the season of Haj in order to suit their own conveniences though it defeated the Divine purpose of Haj and of the forbidden months. It meant that they had rebelled against Allah and become independent of Him; this is kufr. Besides this, Islam is a universal religion and for all human beings. It is obvious that the periods of fasting and performing Haj cannot satisfy all if they be fixed in accordance with the months of the solar year. For if they are performed in fixed months according to the solar year, the one and the same month will not equally suit all for the simple reason that it will fall during the same seasons in different places-summer or winter or very hot or very cold or rainy or dry or harvesting or sowing-year after year.
In this connection it may also be kept in view that the proclamation about the prohibition of "nasi " was made on the occasion of Haj in A. H . 9 and next year,the Haj was performed on right dates which were in accordance with the lunar year. Since that time Haj is being performed according to exact dates.