What is the reward for fasting the day of Arafah and the day of Ashura, along with a day before or after it, and how many years does Allah forgive for the one who fasts them?

Fasting on the Day of Arafah atones for two years: the past and the coming. Fasting on the Day of Ashura atones for the past year. As for fasting on the Day of Tasu'a, its reason is the Prophet's command (peace be upon him) to fast it in order to differ from the Jews and Christians, and its fasting is included in the general virtue of fasting in the month of Muharram.