I made a vow to Allah, the Exalted, that I would fast. It was a long time ago and now I cannot remember the exact number of days that I had vowed to fast. What should I do?

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 is His slave and Messenger.

When one makes a vow to perform an act of worship such as fasting, then he should fulfill it. ‘Aa’ishah reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to observe an act of obedience to Allaah, the Exalted, must do so…" [Al-Bukhari]

Now that you have made a vow to fast a number of days but cannot remember the exact number, you have to do your best to remember how many they are. If you failed to remember the exact number, then you should fast a number of days that makes you almost certain that you have cleared your liability and fulfilled your vow.

This is so if you made a vow to fast a set of unspecific days. However, if you vowed to fast on particular days and failed to deliver on your vow (and you did not fast on those particular days) without an excuse, then you should make up for those days and offer expiation like for breaking an oath. If you failed to deliver on your vow and to fast on those days due to a valid excuse such as illness, travelling, forgetfulness, or menstruation, then (know that the) scholars held different opinions in this regard. The preponderant opinion is that it is incumbent on you to just make up for those fasts.

Allaah Knows best.