The apostle of Allah ﷺ used to fast three days every month: Monday, Thursday and Monday in the next week. (Using translation from Abū Dāʾūd 2451)
كَانَ رَسُولُ اللهِ ﷺ يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنَ الشَّهْرِ الِاثْنَيْنِ وَالْخَمِيسَ وَالِاثْنَيْنِ مِنَ الْجُمُعَةِ الْأُخْرَى