There are five days during the year on which it is forbidden to fast.
These five days are:
The two days of Eid and the three days of Tashreek viz. 11th, 12th and 13th Zil-Haj. (The 3 days following Eid-ul-Adhaa)
Fasting on these days is not valid.
YAUMUSH – SHAKK (THE DOUBTFUL DAY)
(1) The day after the 29th of Sha’ban is termed Yaumush Shakk or the Doubtful Day because of the possibility of that day being the 30th Sha’baan or the 1st Ramadhan.
(2) It is Makrooh to fast on Yaumush Shakk whether it is a Nafl, Qadha or Nathr Saum.
(3) If one’s usual day of Nafl fasting coincides with Yaumush Shakk, it will then not be Makrooh to fast on that day, e.g. it is one’s usual practice to fast on Mondays and Thursdays. Should Yaumush Shakk coincide with Monday or Thursday and one makes the firm intention of Nafl fast, then it will not be Makrooh. Should it be confirmed later that the day is in fact the first day of Ramadhan, the fast thus kept will be regarded as the Fardh Saum of Ramadhan even though the niyyat was for Nafl.
(4) It is Makrooh Tahrimi (which is a forbidden and sinful act) to fast on this day (Yaumush Shakk) with the niyyat of Ramadhan or some other compulsory Saum such as Qadha or Nathr. Nevertheless, if it transpires that the day is in fact the first day of Ramadhan, then this fast will be regarded as the Saum of Ramadhan.
(5) It will be afdhal (best) for a person whose usual day of Nafl Saum coincides with Yaumush Shakk to fast on that day (viz. Yaumush Shakk).
(6) On the Day of Doubt people should abstain from eating until Nisfun-Nahar (about one hour before Zawwal). If by this time news confirming the sighting of the moon is received, they should make the niyyat for Ramadhan Saum. If by Nisfun-Nahar such confirmation is not received, they should eat, for then it is not Ramadhan.