Sheikh Bin Baaz was asked:
Is it permissible for an individual to make up the fasts of the days of the full moon (the 13th,14th and 15th of the month)? For example someone begins to fast them regularly, then something comes that restrains them from doing so.
The Sheikh replied;
It is legislated for the believe man and woman, that they can fast three days from every month. If they fast them during these three days than it is the best situation possible. Although, if they fast them in any other days of the month, then it’s ok for them as well. This is because the Prophet (صلى الله عليه و سلم) advised with fasting three days out of every month. He (صلى الله عليه و سلم) only explained that if the fast was done during these three days, then it is more preferable than fasting them on other days.
Therefore, if a man or a woman fasts these three days, then something busies them from continuing this habit, it is with out a doubt permissible for them to fast during the rest of the month. This is not considered to be making up the fast, because the whole month is considered to be the correct time to fast (from the beginning of the month to the end of it). So, if someone were to fast these three days in the beginning the middle or the end of the month then they have completed that which was legislated from the Sunnah, regardless of whether or not it took place during these three days of the month.
[Taken from Majmoo’ al Fataawah by Sheikh Bin Baaz]
[Translated by Abu Abdillah Abdul Lateef]