question #1 Sahih al-Bukhari 1592, Book 25, Hadith 78 : What was the significance of fasting on the day of ‘Ashura before the fasting of Ramadan became obligatory?

The people used to fast on ‘Ashura (the tenth day of the month of Muharram) before the fasting of
Ramadan was made obligatory. And on that day the Ka`ba used to be covered with a cover. When
Allah made the fasting of the month of Ramadan compulsory, Allah’s Messenger (ﷺ) said, “Whoever wishes
to fast (on the day of ‘Ashura’) may do so; and whoever wishes to leave it can do so.”