Shakk means doubt. The Day of Doubt refers to the day on which there is uncertainty as to whether Ramaḍān has begun—that is, the thirtieth day of the month of Shaʿbān. When there is doubt as to whether Ramaḍān has started, that day is regarded as the Day of Doubt. One might consider fasting on that day as a precaution; however, the Messenger of Allah ﷺ prohibited doing so.
In this regard, the following reports are transmitted from the Messenger of Allah ﷺ:
“Whoever fasts on the Day of Doubt has disobeyed Abū al-Qāsim [the Messenger of Allah].”
(al-Tirmidhī, Ṣawm, 3)
“None of you should fast one or two days before Ramaḍān, unless he has a fast that he has been accustomed to observing.”
(al-Nasāʾī, Ṣiyām, 31)
From the second narration, it is understood that if a person habitually observes a regular voluntary fast (such as the Fast of Dāwūd) and that fast coincides with the Day of Doubt, there is no objection to fasting on that day.
The prohibition of fasting on the Day of Doubt may be based on several underlying wisdoms, including ensuring that Muslims enter the month of Ramaḍān with strength and vitality, encouraging careful verification of the beginning and end of acts of worship, and preventing such precautionary fasting from becoming a fixed practice that could lead to uncertainty regarding the actual beginning of Ramaḍān.