All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.{C}
If it is one's habit to fast every other day, it is not disliked for him to fast on Friday, Saturday or Sunday. It is possible to reconcile the Hadeeth which states that it is forbidden for him to fast any of those days alone and the Hadeeth which states that the fasting of Daawood (David) is the dearest to Allaah, although it would inevitably lead to fasting on Friday, Saturday or Sunday alone, by arguing that the forbiddance is only for the one who voluntarily fasts Fridays or Saturdays specifically, while it is not his habit to fast every other day. However, the one whose habit is to do so, is not included in such forbiddance.
In his book “Al-Mughni”, Ibn Qudaamah from the Hanbali school of jurisprudance said, "According to our companions (scholars in the Hanbali school of jurisprudence), it is disliked to fast Saturday alone…and what is disliked is to fast it specifically; and if one fasts another day with it, it will not be disliked based on the Hadeeth narrated on the authority of Abu Hurayrah and Juwayriyah . Also, if it coincides with the person's habit of fasting, then it is not disliked." [End of quote]
An-Nawawi from the Shaafi'i school of jurisprudence said, "It is also disliked to fast Friday, Saturday or Sunday specifically in view of the statement of the Prophet, sallallaahu ‘alayhi wa sallam: "None of you should fast on Friday alone, unless he fasts a day before it or after it." [Al-Bukhari and Muslim] In another Hadeeth he, sallallaahu ‘alayhi wa sallam, said: "Fast not on Saturday unless it is included in the fasts enjoined upon you." [At-Tirmithi graded it Hasan; and Al-Haakim graded it Saheeh (authentic) according to the conditions of authenticity set by Al-Bukhari and Muslim] Furthermore, the Jews sanctify Saturday, and the Christians sanctify Sunday; and the dislikeability relates to fasting on either day specifically – unless it coincides with one's habit of fasting, such as being used to fast every other day, and his fasting coincides with one of those days – in this case, it is not disliked…" [End of quote]
Allaah Knows best.