Fasting the Day of Doubt
Ammaar ibn Yaasir (radi Allahu anhu) said, “Whoever fasts on the day concerning which there is doubt (the 30th of Sha’baan) has disobeyed Abu’l-Qaasim (the Prophet Muhammad (sal Allahu alaihi wa sallam)).”
[Al-Tirmidhi (686) and al-Nasaa’i (2188) - Classed as Sahih by Sh al-Albaani]
Al-Haafiz Ibn Hajr (may Allah have mercy upon him) said in Fath al-Baari:
It was understood from this (the above Hadith) that it is haram (prohibited) to fast the day of doubt, because the Sahaabah (companions of the Prophet) would not say such a thing based on personal opinion. The day of doubt is the thirtieth of Sha’baan. If the new moon cannot be seen because of clouds etc, it is called the day of doubt because it could be the thirtieth of Sha’baan, or it could be the first day of Ramadan. It is haram to fast on this day, unless it coincides with a day on which a person habitually fasts.
Imam Al-Nawawi (may Allah have mercy upon him) said in al-Majmoo’ concerning the ruling on fasting the day of doubt:
If a person observes a voluntary fast on this day, and he has a reason for doing so, such as having the habit of fasting every day, or of fasting alternate days, or fasting on particular days such as Mondays, and it happens to coincide with that day, then it is permissible for him to fast on this day; there is no difference of opinion among our companions regarding that. The evidence for that is the Hadith of Abu Hurayrah (radi Allahu anhu): “Do not anticipate Ramadan by fasting one or two days before it begins, but if a man habitually fasts, then let him fast.” If he does not have a reason, then it is haram for him to fast on this day.
And Allah knows best!