Ibn `Abbas (may Allah be pleased with them both) said: “I never saw Allah’s Messenger (peace and blessings be upon him) so keen to fast any day and give it priority over any other than this day, the day of `A’shora’, and this month, meaning Ramadan.” (Al-Bukhari, Hadith 1867).
It is preferred to fast the day of A’shora along with one day before it or one day after it or both.
May Allah accept our fast, and He knows best.