The Messenger of Allah ﷺ used to command us to fast on the day of Ashura, urge us to do so, and check on us regarding it. But when the fast of Ramadan was made obligatory, he no longer commanded us to fast it nor checked on us regarding it.
كان رسول الله ﷺ يأمرنا بصيام يوم عاشوراء، ويحثنا عليه، و يتعاهدنا عنده فلما فرض رمضان لم يأمرنا ولم يتعاهدنا عنده.