question #3 Sahih Muslim 1125c, Book 13, Hadith 145 : What options do Muslims have regarding fasting on the day of Ashura in Islam?

‘A’isha (Allah be pleased with her) reported. In the pre-Islamic days fast was observed on the day of Ashura, but with the advent of Islam (its position was ascertained as that of a voluntary fast). Then he who wished to fast fasted, and he who liked to abandon it abandoned it.