Does a permissible fast of a month starting mid-lunar cycle necessitate completing thirty days?
General Chapter
Al-Mughni
Book of Vows
Primary text
If one begins fasting a month from the beginning of the lunar crescent, it suffices whether it is deficient or complete, because the time between two crescents constitutes a month, as indicated by the Hadith stating the month is twenty-nine days. If the fast begins in the middle of a month, thirty days by count are obligatory, based on the saying of the Messenger of Allah (peace be upon him): 'Fast upon seeing it [the crescent] and break your fast upon seeing it; if it is obscured to you, complete thirty.' If Shawwal is fasted, completing thirty days is obligatory because it was started mid-cycle; if it was deficient, two days must be made up; if complete, one day must be added.
Supporting text
If Dhu al-Hijjah is fasted, breaking the fast for Eid al-Adha and the days of Tashreeq does not interrupt continuity, similar to a woman breaking her fast due to menses, and expiation is due. Four days must be made up if the month was complete, and five if deficient. Another potential ruling is that only four days need be made up even if deficient, because the month was started from the beginning, requiring only the missed portion to be made up.