It marks the day in the 7th century when Imam Hussein, the grandson of the Profit Mohammad, along with his family members and close allies, were killed in the battle of Karbala, in present-day Iraq.
Hussein's faith, bravery, and dedication to his companions has been the inspiration for many Iranians through the centuries. Every year, for ten days leading to Ashura, people attend mourning ceremonies and rituals to commemorate Imam Hussein's martyrdom.
Passion plays reenacting his death, called Tazyah, are performed across the country in special auditoriums or in the streets, squares and parks.
Observers vow to donate food and drink to the procession mourners. The food and drinks have to be prepared using the best available products. Across Tehran, queues for the sought-after food items get longer every year, but the high price of the delicacies has not reduced the number of people who take part.
In addition to its deep religious significance, Ashura is part of the Iranian national identity.
The admiration and faith in Hussein among Iranians is so intense that, in spite of all the security issues in traveling to Karbala, each year thousands of men and women travel into Iraq to attend the Ashura ceremonies. Some of these pilgrims have been killed or injured by terrorist attacks on Shia holy shrines, the latest just last week in the town of Kadhymia.
The history and philosophy of Ashura has become a symbol for the struggle against the "oppressors" of the day in Iran. Iranians from across the country staged demonstrations against the Shah's regime on Ashura in 1978 which led to the Islamic revolution.
On the night of Ashura the mourners hold a prayer vigil. This year, groups of people gathered in Palestine Square, in central Tehran, to express their support for the Palestinians. Gaza has dominated this year's Ashura.
"Death to Israel," they chanted, "Today is Ashura and Gaza is Karbala."