Two Textual Bases, One Date, and What Each Tradition Mourns or Marks

Ashura falls on the tenth of Muharram, the first month of the purely lunar Hijri calendar, and the tabular reckoning this rule uses is a fixed arithmetic table rather than an observed crescent, a civil convention that can put the date a day off from wherever a local moon sighting actually governs practice. What the date commemorates differs sharply by branch. Sunni tradition grounds the fast in a report that Muhammad found the Jews of Medina fasting on this day in remembrance of the Exodus and adopted a parallel voluntary fast for his own community, so for most Sunni Muslims Ashura is a day of optional fasting believed to expiate the sins of the preceding year, observed quietly and without mourning.

For Shia Muslims the same date carries an entirely different weight. It is the day in 680 CE that Husayn ibn Ali, grandson of Muhammad, was killed with most of his household at Karbala in present day Iraq, refusing to submit to the Umayyad caliph Yazid. Ashura is accordingly the central date of the Shia liturgical year, marked with mourning processions, recitation of the Karbala narrative, and in many communities passion plays called taziyeh reenacting the battle. The same tabular date thus opens onto two historically distinct observances, a voluntary fast on one side and the deepest communal mourning on the other, joined only by the calendar rule that places them both on Muharram ten.