Islam as a religion does not have a scripturally sanctioned calendar, nor is there a Sunnah or a Prophetic tradition suggesting one.  In Pre-Islamic Arabia there was a traditional 12-month calendar based on lunar phases with an intercalary month, referred to as an-nasī,ʾ ‘postponing’.  The purpose of the intercalary month was to synchronize the lunar calendar with the solar cycle in order to achieve a more consistent relationship between the months and the seasons of th...