[1] There are many interpretations of this Ijadith, but the most common one is this: The good deeds done in these two months are rewarded fully whether these months are of 29 or 30 days. There is no harm if Muslims unknowingly start observing Saum (fast) on a wrong day, or stay at 'Arafat during Hajj on a day other than the prescribed one, on condition that the crescent-moon should be watched carefully. For example, if two persons witness that they have seen the crescent-moon and the people observe Saum (fast) or stay at 'Arafat accordingly, and later the two witnesses turn to be liars, the acts of worshipping performed by the Muslims will not be rejected by Allah. (Fath Al-Bari, Vol. 5, Pages 26,27).