Question: What is the ruling on observing the optional Sawm (fast) of the Day of `Arafah (9th of Thul-Hijjah) while one has yet to make up for days of Sawm missed during Ramadaan?
Answer: If someone observes Sawm on the Day of `Arafah voluntarily while one has to make up for days of Sawm missed during Ramadaan, his Sawm is valid. However, it is encouraged by Sharee`ah not to delay compensation because one’s soul is in Allaah’s Hands and one does not know when death will come. Thus, observing Sawm on the Day of `Arafah as compensation for a missed day of Ramadaan is better than observing voluntary Sawm on that a day. Undoubtedly, obligatory acts of worship are to be given priority over supererogatory acts and should be given greater attention… read more here.