1255/1 - Abu Qatādah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) was asked about fasting Mondays, and he said: “That is a day on which I was born and I was sent as a prophet, or revelation was sent to me.” [Narrated by Muslim]
1256/2 - Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: “Deeds are presented (to Allah) on Mondays and Thursdays. So, I like my deeds to be presented while I am fasting.” [Narrated by Al-Tirmidhi; he classified it as Hasan (sound)]. Muslim narrated it without mentioning the fast.
1257/3 - ‘Ā’ishah (may Allah be pleased with her) reported: “The Messenger of Allah (may Allah’s peace and blessings be upon him) was keen to fast on Mondays and Thursdays.” [Narrated by Al-Tirmidhi; and he classified it as Hasan (sound)]
1) They point out the merit of Monday due to great events that happened on a Monday, namely the birth of the Prophet (may Allah’s peace and blessings be upon him), sending down the revelation, and presenting people’s deeds to Allah Almighty.
2) We ought to be keen to fast Mondays and Thursdays, as our deeds are presented to Allah Almighty on these two days.
3) We are encouraged to follow the Prophet’s noble example, as he used to seek Allah’s pleasure through fasting.
In virtuous days, we should perform acts of worship according to the Prophet’s guidance, the best guidance. We may not perform acts of worship during those days not legislated by Allah Almighty or His Messenger. We are required to adhere to the Prophet’s Sunnah and abandon religious innovations.
‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) said: “Be followers, and do not introduce new things in religion. You already have what is enough. Indeed, every religious innovation is misguidance.” [Narrated by Al-Tabarāni in Al-Mu‘jam al-Kabīr]