Is a vow to fast on the day a specific person arrives valid?
General Chapter
Al-Mughni
Book of Vows
Primary text
The vow to fast on the day a specific person arrives is valid. This is the position of Abu Hanifa and one narration of Al-Shafi'i. The evidence for validity rests on the fact that the day is a time during which voluntary fasting is permitted, thus the vow for fasting on that day takes effect, similar to one becoming a voluntary faster upon waking.
Supporting text
One opinion of Al-Shafi'i states that such a vow is not valid because it becomes impossible to fulfill after the condition (arrival) has occurred, similar to vowing to fast the day before the arrival day. However, the rebuttal is that the arrival day might become known the night before, allowing for intention, and the scenario where obligation becomes impossible (like a minor reaching maturity during Ramadan or a menstruating woman purifying herself) is not analogous to this case.