Q. Is it okay to fast on the day of Aashura/10th of Muharram only?
A. It is Mustahab (preferable) to keep the fast of Aashura in the following manner:
1- To fast on the 9th and 10th of Muharram together
2- To fast on the 10th and 11th of Muharram together
3-To fast on the 9th, 10th and 11th of Muharram all together
However, it is Makrooh (disliked) to keep fast on the 10th of Muharram (the day of Aashura) only. (Tabyeenul Haqaaiq 1/332) This is to distance oneself from enacting the practice of the Jews who fasted on the 10th of Muharram only.
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa