Fasting on Mondays and Thursdays or on Three Days of each Month?
Question

Which is better: fasting three days of each month or fasting on Mondays and Thursdays? What are the ahadeeth which have been narrated concerning these fasts? What are the three days of each month – are they the 13th, 14th and 15th of each month? What is the evidence?
Praise be to Allah.Which is better, fasting on Mondays and Thursdays or on three days of each month?
If we want to discover which is better, fasting on Mondays and Thursdays or fasting three days of each month, we find that fasting on Mondays and Thursdays is better than fasting on three days of each month. This is because if a person fasts on Mondays and Thursdays each week, that means that he is fasting eight days each month, so he will have done both: fasted on Mondays and Thursdays and fasted three days of the month. 
The three days of each month may be fasted at the beginning of the month, in the middle or at the end, and may be done separately or consecutively. But it is better to fast on the ayyam al-beed, which are the days on which the moon is full, namely the 13th, 14th and 15th of each lunar month. 
Virtues of fasting on Mondays and Thursdays
There follow a number of ahadeeth which encourage fasting on Mondays and Thursdays: 
1 –Abu Qatadah al-Ansari narrated that the Messenger of Allah (peace and blessings of Allah be upon him) was asked about fasting on Mondays. He said: “On that day I was born, and on it the Revelation came to me.” (Narrated by Muslim, 1162)
2 – ‘Aishah (may Allah be pleased with her) said: The Prophet (peace and blessings of Allah be upon him) was keen to fast on Mondays and Thursdays. (Narrated by al-Tirmidhi, 745; al-Nasai, 2361; Ibn Majah, 1739; classed as saheeh by al-Albani in Saheeh al-Targheeb, 1044) 
3 –Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Deeds are shown (to Allah) on Mondays and Thursdays, and I like my deeds to be shown when I am fasting.” (Narrated by al-Tirmidhi, 747; classed as saheeh by al-Albani in Saheeh al-Targheeb, 1041) 
Virtues of fasting on three days of each month
The following ahaadeeth encourage fasting on three days of each month: 
1 –Abu Hurayrah (may Allah be pleased with him) said: “My close friend [the Prophet (peace and blessings of Allah be upon him)] 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, 1124; Muslim, 721)
2 –Mu'ahadah al-‘Adawiyyah narrated that she asked ‘Aishah, the wife of the Prophet (peace and blessings of Allah be upon him), “Did the Messenger of Allah (peace and blessings of Allah be upon him) 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.” (Narrated by Muslim, 1160)
3 –Jareer ibn ‘Abd-Allah (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “Fasting three days of each month is fasting for a lifetime, and ayyam al-beed are the thirteenth, fourteenth and fifteenth.” (Narrated by al-Nasai, 2420; classed as saheeh by al-Albani in Saheeh al-Targheeb, 1040) 
4 –Abu Dharr (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: “If you fast any part of the month, then fast the thirteenth, fourteenth and fifteenth.”  (Narrated by al-Tirmidhi, 761; al-Nasai, 2424; classed as saheeh by al-Albani in Saheeh al-Targheeb, 1038) 
What days of the month should you fast?
There is broad scope in the command to fast three days, as it says in the hadeeth of ‘Aishah (may Allah be pleased with her), but the best days of the month for fasting are the thirteenth, fourteenth and fifteenth, as it says in the other saheeh ahaadeeth. 
Shaykh Muhammad ibn Salih al-‘Uthaymeen (may Allah have mercy on him) was asked: When fasting three days of every month, is it essential that it be only on the ayyam al-beed? Or is it permissible to fast any three days of the month? 
He replied: 
It is permissible to fast at the beginning of the month, in the middle or at the end, on consecutive days or separately. But it is better if it is done on the three ayyaam al-beed, which are the thirteenth, fourteenth and fifteenth. ‘Aishah (may Allah be pleased with her) said: The Prophet (peace and blessings of Allah be upon him) used to fast on three days of every month, and he did not mind whether he fasted at the beginning or at the end of the month.” (Majmoo’ Fatawa al-Shaykh Ibn ‘Uthaymeen, 20/question no. 376)
For more on fasting Mondays and Thursdays, please see these answers: 5415 , 106468 , and 109217 .
And Allah knows best.