Is the fast of Ashura Sunnah, and how can it be reconciled with the doubts raised regarding its origin, the determination of its day in the Hijri calendar, and the timing of the Prophet’s (peace be upon him) command for the Jews in Medina to fast it?

Fasting the day of Ashura is a confirmed Sunnah, agreed upon by consensus. Doubts raised to question it are baseless and stem from ignorance of the Jewish reality, for the Hebrew calendar relies on the moon, even if there are additions and modifications. The Prophet, peace and blessings be upon him, was a Prophet who received revelation, and his fasting of it as the day of Moses' salvation is true. Allah made the crescent moon a criterion for determining times. Disagreeing with the Jews in their calculations does not harm, because they altered the laws. The Jews' celebration of Passover now confirms their connection to the day of Moses' salvation. Even if they do not celebrate Ashura today, it does not invalidate the hadiths about their fasting it previously. There is no contradiction in the hadiths: The Prophet, peace and blessings be upon him, fasted Ashura upon his arrival in Medina, then commanded the fasting of the ninth before his death, in order to differ from the Jews, and he died before he could fast it.