4 – Forming the intention during the night is required only for obligatory fasts: the fast of Ramadan in its proper time and when making it up, as well as expiatory and vowed fasts.
5 – A voluntary fast, by contrast, is valid with an intention made during the day, as shown in Hadith 545. One need not set the intention the night before; any time during daylight hours suffices, even after the sun has passed the meridian.
In al-Sharḥ al-Iqnāʿ it is stated: “A supererogatory fast becomes valid with an intention formed during the day, whether before or after the zenith, on the strength of the ʿĀʾishah report.”
6 – The fast earns its legal status and reward from the moment the intention is made, because before that there was no resolve to draw near to Allah, and so no act of worship existed. It is, however, a condition that nothing nullifying the fast was done between dawn and the moment of intention; should a breaker of the fast occur, the fast is invalid by unanimous agreement.
7 – One may form the intention for either an obligatory or a voluntary fast at any time during the night, and if, after intending, he does something incompatible with fasting, the forthcoming fast remains valid so long as dawn has not yet broken.
8 – Al-Sharḥ al-Iqnāʿ further states: “One must specify in his intention the fast he is performing—whether it is a day of Ramadan, a make-up day, a vowed fast, or an expiatory fast—because of the ḥadith, ‘Actions are only by intentions.’ The intention distinguishes acts of worship from one another, and such specification is required in its own right.”
9 – Hadith 545 also shows that completing a voluntary fast is not obligatory; a person may discontinue it and break the fast, though completing it is recommended.
Al-Sharḥ al-Iqnāʿ remarks: “Whoever embarks on a voluntary act of worship other than Hajj or ʿUmrah is recommended to complete it, for perfection of worship is what is sought; yet completion is not obligatory, according to the ʿĀʾishah ḥadith. It is disliked to abandon it without excuse, for that forfeits reward. If he invalidates the fast, no make-up is required, nor do charity, Qurʾan recitation, or remembrances become binding simply because one has begun them, according to the four Imams. If he breaks off a voluntary act of worship, is he rewarded for the portion…"