Allah, Exalted is He, decreed that the sighting of the new moon (crescent) should be usedto determine the timings of the lunar month. The sighting of the crescent marks the end of one lunar month and the beginning of another. Based on this sighting, many religious obligations are determined, such as fasting and Hajj. In this hadeeth, ‘AbdullahIbn ‘Umar (may Allah be pleased with him)related that the Messenger of Allah ﷺ told them that the month may be twenty-nine or thirty days, and that both scenarios arepossible and valid, but determining the timings to observe or break fasting requires the sighting of the moon, and this is the meaning of his saying: “Do not fast until you see the moon,” meaning, ‘do not fast until you see the crescent after sunset of the twenty-ninth day of Sha‘baan.’ He ﷺ added: “If the sky is overcast, then complete Sha‘baan as thirty days.” If the crescent is not seen for some reason - such as clouds and the like - then the month of Sha‘baan is thirty days. It is inferred from the hadeeth that it is not allowable to rely on means other than the sighting of the new moon to determine the beginning and end of lunar months, such as astronomical calculation. .