What the Quran says
The Quran says that the number of months with Allah is twelve, and that four of them are sacred (Quran 9:36). The same verse tells believers not to wrong themselves during these months.
These months were respected by the Arabs before Islam, and Islam confirmed their sanctity. Some Arabs used to postpone a sacred month to suit their wars, and the Quran condemned this practice (Quran 9:37). The verse reminds us that the calendar is set by Allah, and that time itself can carry a special status.