Fasting on the day of Ashura is Makrouh (Disliked). It is called Fasting of Ibn Marjana according narrations from Ahlul Bayt (AS). It is recommended not to eat on the day of Ashura till late afternoon or evening.
'Fasting of day of Arafa is recommended unless it makes you weak to perform the recommended worshiping of the day of Arafa.
Wassalam.