I forgetfully ate while I was fasting a day in place of a day that I missed in Ramadan. I was told that my fasting was invalid and that I did not have to resume fasting the rest of the day because I had already eaten, even though I was forgetful when I ate. I was also told that fasting the days missed in Ramadan is different from fasting in Ramadan itself as regards my situation. Is this true? Another question: Is it permissible to fast the day of doubt?

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

Making up for missed days of Ramadan is the same as fasting in Ramadan itself. So if someone who is making up for a missed fast forgetfully breaks his fast, then he must continue to fast the rest of the day, regardless of what type of fast he is doing. The Prophet, sallallaahu ‘alayhi wa sallam, said, “Whoever forgetfully eats while he fasting, let him complete his fast for it is Allaah Who has provided him with food and drink.” [Al-Bukhari and Muslim; this is the wording of Al-Bukhari]

With this in mind, if you resumed your fasting after forgetfully having eaten, then you have fulfilled your obligation. However, if you broke your fast for the rest of the day, then you are still required to make up for the day of fasting that you missed in Ramadan.

 As for your question about the fasting of the day of doubt, if you mean the last day of Sha‘baan, then such fasting is impermissible according to the reported prohibition thereof. ‘Ammaar Ibn Yaasir said, “Whoever fasts the day of doubt has disobeyed AbulQaasim (The Prophet, sallallaahu ‘alayhi wa sallam).” [Al-Bukhari]

In Fat-h Al-Baari, Ibn Hajar said:

This statement indicates the prohibition of fasting on the day of doubt. This is because a Companion of the Prophet, sallallaahu ‘alayhi wa sallam, would not have said so on his own accord, so it is effectively attributed to the Prophet, sallallaahu ‘alayhi wa sallam. Ibn ‘Abd Al-Barr said in relation to this, “This statement is attributed to the Prophet, sallallaahu ‘alayhi wa sallam, and they all agree upon this fact.”

However, if you are not asking about the fasting of the day of doubt but whether a fast whose validity is doubted can be ruled as fulfilling the obligation, then the answer is that an obligatory fast cannot be valid unless it is evidently observed in the right manner. In other words, an obligation is not fulfilled unless one makes sure that such an obligation is fulfilled as due.

Allaah Knows best.