That was a day on which the people of pre-Islamic days used to observe fast. So he who amongst you likes to observe fast should do so, and he who does not like it should abandon it.
أَنَّهُ ذُكِرَ عِنْدَ رَسُولِ اللَّهِ ﷺ يَوْمُ عَاشُورَاءَ فَقَالَ رَسُولُ اللَّهِ ﷺ كَانَ يَوْمًا يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَصُومَهُ فَلْيَصُمْهُ وَمَنْ كَرِهَ فَلْيَدَعْهُ