Fast on sighting it -the new moon- and break your fast on sighting it. If it is obscured -due to heavy clouds or bad weather, count the next day as the thirtieth -of the current month.

It was said to 'A'ishah: O Mother of the Believers, -the new moon of- this month was sighted on the twenty-ninth day. She said: What amazes you about that? I fasted with the Messenger of Allah (PBUH) more twenty-nine-day months of Ramadan than thirty-day months.

Um Al-Fadhl sent him - Kureib- to Mu'awiyah in the Levant. Kureib said: I went to the Levant and fulfilled her need. The new moon of Ramadan was sighted while I was in the Levant and I saw the new moon on Friday night. Then I came to Medina at the end of the month, and 'Abdullah ibn Abbas asked me about the new moon and said: When did you sight it? I said: We sighted it on Friday night. He said: You sighted it on Friday night? I said: Yes, and people sighted it, so they fasted, and Mu'awiyah fasted. He said: But we sighted it on Saturday night, so we will continue fasting until we complete thirty days or we sight it. So I said: Isn't it enough that Mu'awiyah and his companions have sighted it? He said: No, this is what the Messenger of Allah (PBUH) commanded us.

A month is twenty nine days, so do not fast until you sight it -the new moon, and do not break your fast until you sight it. If it is obscured -due to heavy clouds or bad weather, then assume/estimate it -through calculations. Nafi’ said: So when the twenty-ninth of Sha'ban had passed, Abdullah Ibn 'Omar would send someone to check it, and if he sights it, then that is it. If not, and neither cloud nor dust obscured the sighting, then he breaks his fasting the next day, but if sighting was obscured by cloud or dust. He fasts the next day.