What is the ruling regarding fasting on the Day of Doubt (Yawm al-Shakk)?
General Chapter
Al-Mughni
Book of Fasting
Primary text
Fasting on the Day of Doubt is prohibited, as established by consensus (Muttafaq 'alayh Hadith). The rationale is that the default status is the continuation of Sha'ban, and one cannot transition to Ramadan based on mere doubt. The general principle is that fasting is required only upon sighting the moon, completing thirty days of Sha'ban, or if an obstruction prevents sighting.
Supporting text
The interpretation that one should fast on the Day of Doubt as a precaution, because preferring to fast a day from Sha'ban rather than miss a day of Ramadan is favored, is based on the action of 'Ali, Abu Hurayrah, and 'Aishah. This precautionary measure is supported by the fact that fasting requires only one witness, while breaking fast requires two.