Ashura (Arabic: عَاشُورَاء, ʿĀshūrāʾ [ʕaːʃuːˈraːʔ]) is a day of commemoration in Islam. It occurs annually on the tenth of Muharram, the first month of the Islamic calendar. For Sunni Muslims, Ashura marks the parting of the Red Sea by Moses and the salvation of the Israelites. Also on this day, it is believed that Noah disembarked from the Ark, God forgave Adam, and Joseph was released from prison, among various other auspicious events having occurred on Ashura according to Sunni tradition. Ashura is celebrated in Sunni Islam through supererogatory fasting and other acceptable expressions of joy. In some Sunni communities, the annual Ashura festivities include carnivals, bonfires, and special dishes, even though some Sunni scholars have criticized such practices.

By contrast, for Shia Muslims, Ashura is a day of mourning as they annually commemorate the death of Husayn ibn Ali, grandson of the Islamic prophet Muhammad and the third Shia imam. Husayn refused on moral grounds to pledge his allegiance to the Umayyad caliph Yazid ibn Mu'awiya (r. 680–683) and was subsequently killed, alongside most of his male relatives and his small retinue, by the Umayyad army in the Battle of Karbala on Ashura 61 AH (680 CE). Among the Shia, mourning for Husayn is viewed as an act of protest against oppression, a struggle for God, and a means of securing the intercession of Husayn in the afterlife. Ashura is observed through mourning gatherings, processions, and dramatic reenactments. In such ceremonies, Shia mourners strike their chests to share in the pain of Husayn. Extreme self-flagellation, often involving self-inflicted bloodshed, remains controversial among the Shia, condemned by many Shia clerics, and outlawed in some Shia communities. Ashura has sometimes been an occasion for sectarian violence, particularly against the Shia minority.

Ashura is an Aramaic word meaning 'tenth'. It may have also been derived from the Syriac words asiroya or asora. It shares the same root as the Hebrew word 'āsōr. In Arabic, Ashura refers to the tenth day of Muharram, the first month of the Islamic calendar, a month in which fighting has been forbidden since before the advent of Islam.

Fasting on Ashura was likely a Jewish practice adopted by the Islamic prophet Muhammad after his arrival in the city of Medina in 622 CE, perhaps signifying Muhammad's sense of a shared prophetic mission with Moses. Although it remained optional, fasting on Ashura ceased to be a religious obligation after about a year when the relations with the Medinese Jews deteriorated. This transition is often associated with verses 2:183–5 of the Quran, the central religious text in Islam, which explicitly designate Ramadan as the month of fasting. It also seems improbable that Ashura initially coincided with the tenth of Muharram. Instead, Ashura was probably observed at first on the tenth of the first Jewish month of Tishrei, known as Yom Kippur (lit. 'day of atonement'). The association of Ashura with the tenth of Muharram thus happened later, some time after the Jewish and Muslim calendars diverged. In turn, the calendars began to diverge when Muhammad forbade Jewish-type calendar adjustments in connection with verse 9:37 of the Quran.

A similar origin story for Ashura appears in some Sunni traditions. Alternatively, there are traditions in canonical Sunni collections that describe fasting on Ashura as a pre-Islamic practice among the Quraysh tribe, in which Muhammad also partook while he was in Mecca. Some early Sunni traditions, many classified as unreliable, possibly invented by the Umayyads (r. 661–750), link Ashura to various auspicious events: On this day, Moses parted the Red Sea, Noah disembarked from the Ark, God forgave Adam, Joseph was released from prison, Jesus, Abraham, and Adam were born, Muhammad was conceived, and Jonah was freed from the fish that had swallowed him. Fasting on the ninth of Muharram, known as Tasu'a, was a later addition, probably to distinguish Muslims from Jews.

In Sunni Islam, ninth and tenth of Muharram are days for voluntary fasting, strongly encouraged by Sunni jurists. While not endorsed by all Sunni scholars, Ashura is further viewed as a day of thanksgiving (shukr) to God, a joyous occasion, celebrated through pious acts and acceptable expressions of delight. Ashura is thus an important festival for many Sunnis, in contrast to the Shia, who mourn on this day the slaughter of Muhammad's grandson, Husayn ibn Ali, and his small retinue in the Battle of Karbala in 680. Such Sunni festivities either developed in response to Shia customs on Ashura or with the influence of pre-Islamic traditions. In line with the former view, under the Umayyad caliph Abd al-Malik ibn Marwan (r. 685–705), Ashura was celebrated as a festive public holiday to counter the commemoration of Husayn. The Abbasid caliph al-Qadir (r. 991–1031) did so too in Baghdad, Iraq. Another instance is the reenactment by a Sunni mob of the Battle of the Camel (656) against Ali ibn Abi Talib, the first Shia imam, in the Buyid-era Baghdad on Ashura 973.

Whatever the case is for their origins, such festivities were firmly established by the time of the Sunni jurist Ibn Taymiyya (d. 1328), to whom a questioner wrote, observing that people are joyful on Ashura; they bathe, adorn themselves, shake hands with each other, and cook grains. In the Maghreb, for instance, Ashura is celebrated today through fasting, almsgiving, honoring the dead, special dishes, jumping over bonfires, and carnivals. Nevertheless, particularly in South Asia, some Sunnis participated in the Shia rituals on Ashura, at least until modern times. Sufis also commonly commemorated the death of Husayn, more so in the earlier times, despite its variance with the views of the Sunni elite. For Sufis, rather than a tragedy, Ashura celebrates the eternal life of Husayn and his companions, who annihilated themselves in the Divine with their martyrdom.

In response to an inquiry about their legal basis, Ibn Taymiyya rejects both mourning and celebrating on Ashura because, he contends, neither was practiced by Muhammad. Ibn Taymiyya does, however, encourage fasting on Ashura to emulate Muhammad. The Islamicist M. Katz questions the judgment of Ibn Taymiyya for not taking into account the Sunni reports that Muhammad fasted to celebrate Ashura, suggesting that Ibn Taymiyya has stripped fasting of its higher meaning.