In the Name of Allah, the Most Gracious, the Most Merciful.
As-Salāmu ‘Alaykum Wa-Rahmatullāhi Wa-Barakātuh.
The Masnoon manner is to fast on the 9th/10th or on the 10th/11th of Muharram, with the former being more virtuous.[1]
It is permissible to fast all three days. According to some, fasting all three days is most virtuous and rewarding.[2]
And Allah Ta’ala Knows Best.
Azhar Mownah
Student Darul Iftaa
Mauritius
Checked and Approved by,
Mufti Muhammad Zakariyya Desai.
https://iftaa.co.za/8387/is-it-permissible-to-fast-on-all-3-of-the-9th-10th-and-11th-of-muharram/