Q: Is it permissible to fast only on 10th of Muharram?

A: This is not advisable though permissible. Preferably, one should fast on two days, either the 9th and 10th or the 10th and 11th. The Hadith advises that we add a day before or after Ashura.

And Allah Knows Best
Darul Ihsan Fatwa Dept.