It is recommended to fast three days of every month, and to fast on the days of 'Arafah and 'Ashura', and to fast on Mondays and Thursdays Sahih Muslim 1162 e
Translation
Abu Qatada Ansari (Allah be pleased with him) reported that Allah's Massenger (ﷺ) was asked about fasting on Monday, whereupon he said
It is (the day) when I was born and revelation was sent down to me.