It is Makruh (discouraged) to fast on Ashura. https://www.imam-us.org/fasting-on-ashura/?utm_term=%5Cu2018Fasting%20on%20the%20Day%20of%20Ashura%5Cu2019&utm_campaign=Exclusive%20Content%20for%20Muharram%2C%201439%20A.H.&utm_content=email&utm_source=Act-On+Software&utm_medium=email&cm_mmc=Act-On%20Software-_-email-_-Exclusive%20Content%20for%20Muharram%2C%201439%20A.H.-_-%5Cu2018Fasting%20on%20the%20Day%20of%20Ashura%5Cu2019