Islam being a religion that is natural, embraces simplicity and is a perfect way of life for all people of all times irrespective of their age, class, creed, nationality or background has made those aspects upon which is dependant the spiritual life of every believer easily understandable, securable and natural, whether one may be rich or poor, lettered or unlettered.
For example: For Salaat, which is incumbent upon every believer, Allah has made the direction of the Ka’bah (Qiblah) a prerequisite and not the facing of the actual Ka’bah. This type of information can easily be secured and established by every believer whether literate or illiterate. Had this not been the case, this would warrant that in order to discharge the obligation of Salaat, one secure proficiency in the respective field? The result would be that only certain sections of the Ummah would be able to discharge the obligation of Salaat.
Similarly, many aspects of a believer’s spiritual life (fasting, Hajj, Zakaat, etc.) are dependant on the Islamic calendar. Therefore, Islam has chosen the actual visual sighting of the moon (Crescent signaling the new month) which can easily be established by every individual of this Ummah.
In the light of the Ahaadith of Nabi [sallallaahu alayhi wasallam] and the references of the Fuqahaa (Jurists), an Islamic month is 29 days. Subsequently, if the moon is sighted, this will signal the beginning of the new month. If the moon is not sighted after 29 days, one more day would be added to the current month after which the new month will commence. (Sahih al-Bukhari Hadith1907)