SHAFAQNA- Shafaqna Dictionary of Shia Islam in Media is a multilingual collection of Shia Graphs that explain the words used in Shia News and Shia Media.
Yawm Al-Shakk
Since every month of the Lunar Calendar is either 29 or 30 days, if on the evening of the twenty-eighth day of a lunar month, the crescent moon (Hilal) is not seen due to cloudy weather, or there is a difference in the sighting of the moon, that day is called Yawm Al-Shakk. Yawm Al-Shakk is often used for suspicious days between the end of Sha’ban and the beginning of Ramadhan.
The same thing goes for the day after Ramadhan 29, which could be either the 30th day of Ramadhan or the 1st day of Shawwal and Eid Al-Fitr, so the doubtful day is considered a part of the previous month (Ramadhan) according to the rule of Istishab, and its fast is Wajib (obligatory). If during the day it became clear that the crescent moon of Shawwal was seen in the evening of the previous night, one should break his fast because fasting on Eid Al-Fitr is Haram (forbidden).
[Video] Commentary of the Holy Quran 32:17