بسم الله الرحمن الرحيم
In the Name of Allah The most Gracious The Most Merciful
Narrated that Abu Hurairah (رضى الله عنه) said: “My close friend [the Prophet (صلى الله عليه وسلم) advised me to do three things which I will not give up until I die: fasting three days of each month, praying Duha, and sleeping after Witr. (Narrated by al-Bukhari)
Narrated from Mu’aadhah al-‘Adawiyyah that she asked ‘Aa’ishah (رَضِيَ اللَّهُ عَنْهَا), the wife of the Prophet (صلى الله عليه وسلم), “Did the Messenger of Allaah (صلى الله عليه وسلم) fast three days of every month?” She said, “Yes.” She said to her: “Which days of the month did he fast?” She said, “He did not mind which days of the month he would fast.” (Sahih Muslim)
Narrated from Jareer ibn ‘Abd-Allaah (رضي الله عنه) that the Prophet (صلى الله عليه وسلم) said: “Fasting three days of each month is fasting for a lifetime, and ayaam al-beed are the thirteenth, fourteenth and fifteenth.” (an-Nasaa’i, 2420; classed as sahih by al-Albaani in Sahih al-Targheeb, 1040)
Narrated that Abu Dharr (رضي الله عنه) said: The Messenger of Allaah (صلى الله عليه وسلم) said to me: “If you fast any part of the month, then fast the thirteenth, fourteenth and fifteenth.” (at-Tirmidhi, 761; an-Nasa`i, 2424; classed as sahih by al-Albaani in Sahih al-Targheeb, 1038)
The best days to fast these three days are ayam al beedh (the White days). These days occur on the 13,14 and 15th of every month in the lunar calendar. The Muslim can still perform them on any three days of the month and they do not have to be consecutive but the best is to still fast them on those days.
Q: Is it permissible for a person to make up for not fasting the White Days (13th, 14th, and 15th of every Hijri month) because when he starts to fast them, certain circumstances hinder him from completing them?
A: It is prescribed for the believers, men and women, to fast three days of every month. It is better to fast them on the White Days. Yet, if a person fasts them during any other days in the month, this will suffice because the Prophet (peace be upon him) prescribed for the Muslims to fast three days of every month and explained that fasting them during the White Days is much better than doing so on any other days. Thus, if a person is accustomed to fasting the White Days but something happened that made them too busy to do so, then it is permissible for them to fast other days throughout the month, praise be to Allah. This cannot be called making up for these days, because the whole month from its beginning to its end is suitable for fasting. Thus, if a believer, man or woman, observes fasting three days at its beginning, middle, or end of the month, then he would be applying an act of Sunnah (action following the teachings of the Prophet) even when fasting them on days other than the White Days. Fatwah of Shiekh ‘Abdul Aziz Ibn baz (رحمه الله)