It was narrated from Hunaydah ibn Khaalid from his wife that one of the wives of the Prophet (peace and blessings of Allaah be upon him) said: ?The Messenger of Allaah (peace and blessings of Allaah be upon him) used to fast nine days of Dhu?l-Hijjah and the day of ?Ashoora? and three days of each month ? the first Monday and two Thursdays.? Narrated by Imam Ahmad, 21829; Abu Dawood, 2437; classed as da?eef in Nasab al-Raayah, 2/180, but classed as saheeh by al-Albaani.
I thought it was recommended to fast every monday and thursday but it seems that if you are going to fast the 13,14,15th of every month, you only fast 'the first Monday and two Thursdays'. Is that the correct understanding or not?