Sheikh Bin Baaz was asked;
Oh Sheikh, it is known that no one knows the exact beginning of any lunar month, so how is it that we calculate the 3 days of the full moon so that we know when we are supposed to fast them?
The Sheikh Replied;
It has been legislated that you you fast them according to the calendar. This is many times based off of that which has almost reached the level of being sure. If you fasted it in other then those days, you will still get the reward for it. This is because the Prophet (صلى الله عليه و سلم) recommended the people to fast it in every month (meaning the three days) and did not restrict it to the days of the full moon. We see this in the Saheeh of Imam al Bukhari and the Saheeh of Imam Muslim that the Prophet (صلى الله عليه و سلم) said to Abdullah bin Amr bin A’as (رضي الله عنهما),
“صم من الشهر ثلاثة أيام ، إن الحسنة بعشر أمثالها ، فذلك صوم الدهر”
“Fast three days out the month. Every good deed is worth ten of its like. This way it is like fasting the whole of the year.”
[Narrated by both Al Bukhari and Muslim]
As well there is a hadeeth in both Saheeh al Bukhari and Saheeh Muslim on the authority of Abu Hurairah (رضي الله عنه), where he said that,
“(أوصاني خليلي صلى الله عليه و سلم بثلاث (صيام ثلاثة أيام من كل شهر وركعتي الضحى وأن أوتر قبل أن أنام”
“My close companion may Allah’s prayers and peace be upon him advised me with three things (fasting three days in every month, the two rakaat of duhaa and to make witr before I sleep).”
[Narrated by both Al Bukhari and Muslim]
The number of hadeeth about this are very many. The person that fasts them are free to do as they please, if they want they can fast them all consecutively, and if they want they can separate between the days by a day or some days that they do not fast. We understand this because of the different hadeeth are general do do not restrict the situation so much as to make it necessary to fast them all together.
[Taken from Majmoo’ al Fataawah by Sheikh Bin Baaz]
[Translated by Abu Abdillah Abdul Lateef]
