The Prophet صلى الله عليه وسلم said regarding the day of Ashura, fasting the day of Ashura with hope from Allah will expiate your sins for the year that came before it/the previous year.
Ashura is on the 10th of Muharram.
The best days to fast is on the 9th, 10th and 11th.
Or you can fast the 9th and 10th.
Or you can fast the 10th and 11th.
Or you can fast just the 10th.
All are acceptable however the best is to fast on the three days (9th, 10th and 11th).
- Shaykh Musa Jibril حفظه الله