Fast when you see it, and break the fast when you see it, but if the sky is overcast and you cannot see it, then act on estimation
Ibn ‘Umar (may Allah be pleased with him and his father) reported: I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "Fast when you see it, and break the fast when you see it, but if the sky is overcast and you cannot see it, then act on…