" Fasting on Arafat if it coincides with FridayExplained by Shaykh ibn Baz
Question: The day of Arafat came on Friday, and I fasted on Friday which was theday of Arafat, but I did not fast on Thursday; so is there any sin upon me?

Shaykh bin Baz: We hope there is no sin upon you. This is because you did notintend to single out this day for fasting; rather you only fasted this day because itwas the day of Arafat. But if you would have fasted Thursday along with it, it would have been safer and less doubtful. 

This is because the Messenger  prohibited singling out Friday for fasting as it relates to the one observing and optional fast. And you wereobserving an optional fast. Therefore if you would have fasted Thursday along with it, with would have been safer and less doubtful even if your intention was to fast because it was Arafat. But for the believer to investigate so he can be in agreement with the commands of the Prophet  is safer and less
doubtful. As for fasting (Friday) intending to do so because it is a virtuous day,then this is not permissible because the Messenger prohibited that. But if you fast this day because it is the day of Arafat then we hope there will be nothing upon you. But if the person takes precaution and fast on Thursday along with Friday,this will be safer. 

Translated by Rasheed ibn Estes Barbee
http://mtws.posthaven.com/fasting-on-arafat-if-it-coincides-with-friday-explained-by-shaykh-ibn-baz "