Then fast the next 10th Muharram (Ashur'a). Because our beloved s.a.w said fasting the day of Ashura'a will erase the sins of the past year. And adding to it the day before or the day after is better so our fast will be different from Jews!
Rasul Allah (صلى الله عليه وسلم) was questioned about Ashura (10th Muharram) Rasul Allah (صلى الله عليه وسلم) said: It erases the previous years sins (Muslim Sharief).
However, the Jews also used to fast on this day, therefore, Rasul Allah (صلى الله عليه وسلم) said that fast one day before Ashurah or a day after it (Musnad Ahmad). Hence, it is Sunnah to fast on the 9th & 10th of Muharram