'Ashurah was a day on which we used to fast in pre-Islamic days. When (fasting of) Ramadan was prescribed, the Messenger of Allah ﷺ said: This is one of the days of Allah ; he who wishes may fast on it. (Using translation from Abū Dāʾūd 2443)
كَانَ يَوْمُ عَاشُورَاءَ يَوْمًا يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَلَمَّا نَزَلَ رَمَضَانُ سُئِلَ عَنْهُ رَسُولُ اللهِ ﷺ قَالَ هُوَ يَوْمٌ مِنْ أَيَّامِ اللهِ تَعَالَى مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ