Islamic months begin and end upon sighting of the Moon. The day after the Moon is sighted is the first day of the following month. The beginning and ending of the months relies on witnessing the Moon sighting. Allah Subhanahu Wa-Ta’ala clearly mentions this in the Holy Qura’an: “They ask you about the new moon. Say you, ‘They are the signs of time for the people and pilgrimage (Hajj).' (Q2:189)