question #1 Sahih al-Bukhari 1893, Book 30, Hadith 3 : What was the significance of fasting on the day of Ashura’ before the advent of Islam?

(The tribe of) Quraish used to fast on the day of Ashura’ in the Pre-Islamic period, and then Allah’s
Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed;
whereupon the Prophet (ﷺ) said, “He who wants to fast (on ‘Ashura’) may fast, and he who does not want
to fast may not fast.”