“With regard to (the hadeeth) ‘Don’t fast on Saturday,’ some people are under the false impression that if we leave off fasting on Saturday due to it coinciding with the Day of ‘Aashooraa or the Day of ‘Arafah or the like, then we would lose the excellence of fasting on one of these two days that coincided with Saturday. No, we would not lose, (rather) we would make two gains.
The first gain is that we have submitted our hearts and souls to the statement of our Prophet (sallallaahu ‘alayhi wa sallam): ‘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’ . Hence we have followed this hadeeth because fasting on the Day of ‘Arafah and fasting on the Day of ‘Aashooraa are not obligatory. The Messenger (‘alayhi ssalaam) only made obligatory fasts an exception – if they coincide with Saturday, it is allowed to fast them, (but) those fasts other than the obligatory ones, we were prohibited from fasting them (if they coincide with Saturday).
The second gain is that we are included in the likes of his (‘alayhi ssalaam) statement: ‘Whoever leaves something for Allaah’s Sake, Allaah will replace it for him with something better'. So we have left fasting on the Day of ‘Arafah due to it coinciding with Saturday. We have not lost the fast of the Day of ‘Arafah. Why? Because we have left it for the sake of Allaah, so Allaah will replace it for us with something better. The same thing is said even more so regarding the fast of the Day of ‘Aashooraa.”
 Saheeh at-Tirmidhee 744, Saheeh Ibn Maajah 1413
 Shaikh al-Albaani graded it authentic in Hijaabul-Mar.ah p.49