‘Āshūrā’, and distinguish yourselves from the Jews: fast a day before it and a day after it.” Reported by Ahmad (1). He also narrated (2): “The Messenger of Allah (peace and blessings be upon him) instructed us to fast ‘Āshūrā’ on the tenth day.” Reported by al-Tirmidhī (3).
The answer to these difficulties, with Allah’s help, guidance, and support, is as follows:
As for the first difficulty—that when he arrived in Medina, he found them fasting ‘Āshūrā’—the report does not say that he found them fasting it on the very day of his arrival. He arrived on Monday, the twelfth of Rabī‘ al-Awwal. Rather, he first learned of this practice, and the events of that day took place, after his arrival in Medina, not while he was in Mecca. This assumes that the People of the Scripture calculated the date of their fast by lunar months. If, however, they calculated it by solar months, as is their known religious practice (4), the difficulty disappears entirely. The day on which Allah saved Moses would then be the day of ‘Āshūrā’, early in Muharram, but the People of the Scripture would have fixed its date according to solar months. It would thus have coincided with the Prophet’s (peace and blessings be upon him) arrival in Medina in Rabī‘ al-Awwal (5), and the fasting of the People of the Scripture