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There are Sunni traditions to the effect that the Prophet on migrating to Medina found the Jews fasting on the 10th of Muharram. He asked them why, and was told: "It is an auspicious day; it is the day when God delivered the children of Israel from their enemy; and, therefore, Moses fasted on that day [because of the broken tablets]." The Prophet said, "I am worthier of Moses [more ‘Jewish’] than you are." Thereupon, he fasted on that day and ordered [the Muslims] to fast. The importance of this fast to the Prophet may be judged from another tradition narrated in al-Sahih of al-Bukhari: "The Prophet ordered a man from the [tribe of] Aslam: Announce to the people that whoever has eaten should fast the rest of the day, and whoever has not eaten should fast [the whole day], because today is the 'Ashura [10th day of Muharram]." In another hadith ‘Ubayd b. 'Umayr said: "Al-Muharram is the month of God. It is the beginning of the year. It is used as the beginning of the era. In al-Muharram, the Ka'bah is clothed, and money is coined. There is one day in al-Muharram on which repenting sinners are forgiven". This “day of forgiveness” on the 10th of the first month of the year was apparently Yom Kippur.

If the Jews had their own calendar and month names as they do now, the “10th of Muharram” would not have been used to describe that day unless Arab Muharram and the Jewish Tishrei were equivalent. But while there seems to be some disagreement as to whether the Prophet kept the Sabbath or not, it is clear that he was not keeping Yom Kippur when he passed by the Jews of Medina who were fasting that day. The Prophet is described as if he were unaware of the fast, yet quite willing to accept it. One possibility is the Prophet was simply unaware of Jewish customs, yet immediately willing to adopt them. This seems improbable. Another possibility is that the Jews and the Prophet were keeping different, yet related calendars (one lunar the other lunar-solar).

The commentator of Mishkatul-Masabih notes that "it was in the second year [623 CE], because in the first year the Prophet had arrived at Medina after Ashura, in Rabi al-awal."

This is an important observation, because it assumes the Prophet kept the same strictly lunar calendar that is in use today. If the Prophet arrived in Rabi al-Awal, he arrived two months after Muharram, the month containing Ashura, and would have to wait until the next year for the above event to take place.

However in that year the 10th of Tishrei fell on the 10th of Rabi al-awal. So another interpretation of the event, one that agrees more readily with the hadith, would be the first day of the Prophet's arrival in Media was Yom Kippur. This would mean that the Prophet's declaration "today is the 'Ashura", amounted to an adoption of the 19 year lunar-solar calendar.

Husayn ibn Ali kept Yom Kippur

In 680 CE, Husayn, the second son of Ali and thus a grandson of the prophet Mohammed, challenged Yazid's right to the caliphate. Encouraged by factions that opposed Yazid I, Husayn marched to Kufa, expecting to be received with honor and enthusiasm. However, Yazid I sent a strong military force that killed Husayn and all his family. Tabari relates in detail, but without explanation, that Husayn was fasting on the day he was killed. It probably of some significance that October 10, 680 (Julian) - the 10th of Muharram 61 A.H., appears to have coincided with Yom Kippur that year.