I intended to fast the thirteenth, fourteenth and fifteenth days, and thinking that they will start from Saturday, I took a bite of food on Friday morning, and before completing it, I came to know that it was the thirteenth of the month. Upon that, I took out what was in my mouth. Should I or should I not complete the fast of that day?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

If the food did not reach your stomach, there is no harm on you to complete the fast of that day as long as you did not nullify it by any fast nullifier such as eating or drinking or so. As far as the voluntary fast is concerned, one is not required to intend fasting from the night before, according to the majority of the scholars; rather, it is permissible to make the intention for it during the daytime. The evidence is taken from the hadith narrated on the authority of ‘Aa’ishah in which she said, “The Messenger of Allah, sallallaahu ‘alayhi wa sallam, entered upon me one day and asked, ‘Do you have anything (to eat)?’ I answered in the negative. On that, he said, ‘Then I am fasting.’” [Muslim and others]

It is narrated on the authority of Umm Ad-Dardaa’ that she said, “It was the habit of Abu Ad-Dardaa’ to ask us whether we had any food, and if we answered in the negative, he would say, ‘Then, I am fasting this day of mine.’” [Al-Bukhari] Al-Bukhari commented, “The same was done by Abu Talhah, Abu Hurayrah, Ibn ‘Abbas, and Huthayfah .”

In his Sharh Al-Muntaqa, Ash-Shawkaani said, “The majority of the scholars, according to An-Nawawi, presented the hadith of ‘Aa’ishah as proof that in the voluntary fast, one is not required to intend fasting from the night before.”

Allah knows best.