30th day of Shaban is called Yamul Shak (doubtful day) if you do not receive news about Ramadan moonsighting on 29th evening or before the end of 30th day.
It is haram to fast on 30th Shaban with the niyat intention of 1st of Ramadan.
It is Mustahab to keep fast on 30th Shaban but the Niyyat (intention) should be either
(a) with the Niyyat of Mafiz-zimma (to discharge my responsibility), or
(b) with the Niyyat of any Qaza fast of previous Ramadan, if any, or
(c) with the Niyyat of Mustahab Shaban fast.
If you do a double Niyyat of fasting (i.e. 30th Shaban or 1st Ramadan), then such a Niyyat is wrong according Ayatullah Khui, but it is OK according to Ayt Khomeini and Ayt Seestani.
If you are fasting on 30th Shaban, and if, at any time on that day, you get the news of moon sighting of 29th Shaban, then you must immediately change Niyyat to Niyyat of 1st of Ramadan.
If you have fasted on 30th Shaban (with any Niyyat), and afterwards you come to know that it was 1st of Ramadan, then your fast will automatically be counted as fast of 1st of Ramadan.
If you are not fasting on 30th Shaban, then following actions are necessary on you:
(a) If you get the news after sunset of 30th Shaban or later that the moon had been actually sighted on evening of 29th Shaban, then you have to keep Qaza of 1st Ramadan after the month of Ramadan.
(b) If you get the news of moon sighting after Zawal time (Islamic mid-day), then it is haram to eat or drink or do anything, which is not allowed during fasting from that time onward until Iftar time on that day, and you have to do Qaza of 1st of Ramadan later on.
(c) If you get the news of moon sighting before Zawal time (Islamic mid-day), then
(i) if you have not eaten or drunk anything or done anything which breaks fast, you must immediately do the Niyyat of fasting of Ramadan for that day,
(ii) if you have eaten or drunk something or have done anything which breaks fast, then you have to act as if fasting for the rest of the day, and then do Qaza of 1st of Ramadan later on.
