Ат-Тавба 9:36

Religious commandments can be carried out by everyone individually. But it is required by Almighty God that all the people of Faith should perform them together, so that a collective spirit is created in them. For the purpose of promoting this collectivism, definite timings and dates have been fixed for the performance of the religious rites of prayers, etc. Had these dates been appointed according to the solar calendar, there could have been uniformity in the time of their occurrence. For example, the days of fasting would always have come in one particular season and likewise Hajj. But, whereas uniformity creates inertia in man, change introduces new inspiration for action. Therefore, the natural lunar calendar has been adopted to provide the basis for the collective pattern of religious matters. On account of this principle, the dates of Hajj (pilgrimage) fall in different seasons —sometimes in summer and sometimes in winter. In ancient times when the congregation for Hajj was most important from the commercial point of view, the occurrence of Hajj in different seasons appeared to be harmful. Worldly considerations appeared to the Arab people more important than religious imperatives. They wanted to adopt a system by which Hajj would fall in one suitable season. At that time they came to know of the adjustments possible in the calculation (kabisa) of the Jews and Christians. They approved of this, as it was exactly according to their wishes, and they therefore adopted it. This meant displacing months and replacing one month by another, for example, replacing the month of Safar by the month of Muharram and vice versa. ‘Fight the polytheists all together, as they fight you all together.’ This means that just as the infidels unite on their refusal to fear God, the Muslims should unite on their fear of God (taqwa): If they join together for negative purposes, the Muslims should join together for positive ends. If they become one for the sake of the world, the Muslims should become one for the sake of the Hereafter.