Only fast Friday if combined with Thursday, but he who unknowingly fasts on Friday alone is then obliged to fast on Saturday – Shaikh al-Albaani

~Reconciling between the two hadeeths:
‘Don’t fast on Saturday unless it is obligated upon you, and even if you don’t find anything but a bark of a tree, chew on it’[1]
&
‘Don’t fast on Friday except by also fasting a day before it or a day after it’[2]~

Shaikh al-Albaani:

“The one who has in his mind this general prohibition of all fasts that fall on Saturdays except those that are obligatory which must be done, we say to him: don’t fast on Friday to thereafter fast on Saturday. As for the one who fasts on Friday (alone) because he doesn’t have knowledge of this prohibition, then we say to him: fast Saturday along with Friday.

There is still the generality of the hadeeth: ‘Don’t fast on Saturday unless it is obligated upon on.’ It does not contradict with the command to fast on Saturday with Friday for the one who has already fasted on Friday. In such a case, his fasting on Friday singly is prohibited, so to be free from falling into this prohibition, he is obligated to fast on Saturday.

Whoever fasts on Friday not knowing that it is forbidden to fast on it alone, then he (also) fasts on Saturday, because he is obliged to fast in this situation. But the one who knows, then he is not allowed to fast on Friday except if he had already fasted on the Thursday before it.”

[1] Saheeh at-Tirmidhee 744, Saheeh Ibn Maajah 1413
[2] Saheeh al-Bukhaari 1985

[silsilatul-hudaa wa-nnoor 495/4 & 669/24 / asaheeha translations]