This is the day of Ashura, so fast on it, for the Messenger of Allah ﷺ commanded that it be fasted.
حدثنا حسين بن محمد حدثنا إسرائيل عن ثوير قال
هذا يوم عاشوراء فصوموا فإن رسول الله ﷺ أمر بصومه
Its isnad is daif, because of Thuwayr, and the hadith has already appeared at 16064.