the Messenger of Allah used to fast three days of each month: Monday at the beginning of the month, the following Thursday, then the Thursday after that. (Using translation from Nasāʾī 2414)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ «يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ يَوْمَ الِاثْنَيْنِ مِنْ أَوَّلِ الشَّهْرِ ثُمَّ الْخَمِيسِ الَّذِي يَلِيهِ ثُمَّ الْخَمِيسِ الَّذِي يَلِيهِ»