Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him).
Is it better to fast Monday and Thursday or the White Days?
If you are able to do so, it is better to fast every Monday and Thursday because if you do so you will fast eight days in every month, as opposed to only three days if you fast the White Days. One of those Mondays or Thursdays is also likely to coincide with the White Days. If you are able to fast the White Days and another five days in addition to this, this would also be good. We cannot say categorically that one type of fasting is better than another. If someone finds that a certain type of fasting has more effect on their heart and helps him on his path to Allah then this is the best fast for that person.