Question: I broke my Sawm (fast) one day of Ramadaan due to severe illness. Is it permissible for me to make up for it on the Day of `Arafah (9th of Thul-Hijjah) – the Day marking the Hajj – bearing in mind that I had already observed Sawm on that day?
Answer: If you have already observed Sawm on the Day of `Arafah to make up for the day you did not fast in Ramadaan, it will replace making up for the day you missed. However, it is preferable to make up for the days of Sawm one misses on a day other than the Day of `Arafah, so that one may dedicate oneself to Thikr (Remembrance of Allaah), Du`aa’ (supplication) and other acts of worship during Hajj or to fast it voluntarily outside of Hajj. By doing so, one may combine both virtues of observing Sawm voluntarily on the Day of `Arafah and compensation reconciling the opinions of scholars regarding whether compensating for days of Sawm missed during the first nine days of Thul-Hijjah is Makrooh (disliked)… read more here.