Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is permissible to combine the intention on fasting the missed days of Ramadan and the first nine days of Dhul-Hijjah, except the ninth day, the Day of Arafah, as fasting this day expiates the sins of both a previous and coming year.
Responding to your question, Dr. Mahmud Al-Khayami, Head of Islamic Studies Dept., Faculty of Medicine, Al-Azhar University, stated:
All Muslims are encouraged to do their best to do good deeds in these best days of the year.
It was narrated that the Prophet (peace be upon him) said, “No days in which the good deeds are better than these days – referring to the first ten days of Dhul-Hijjah.” (Al-Bukhari)
It is permissible to combine the intention on fasting the missed days of Ramadan and the first nine days of Dhul-Hijjah, except the ninth day, the Day of Arafah, as fasting this day expiates the sins of both a previous and coming year.
It was narrated that the Prophet (peace be upon him) when asked about fasting the Day of Arafah said, “I hope it expiates the sins of the previous and the next year.” (Muslim)
Therefore, you should fast this day with a particular intention, not combining any intention with it, so that you can get the great reward which the Prophet promised in the above hadith.
Almighty Allah knows the best.