Arba'een is a Shia Muslim religious observance which is celebrated forty days after the Day of Ashura, falling on the 20th or 21st day of the Islamic month of Safar.
It commemorates the martyrdom of Muhammad's grandson Husayn ibn Ali.
Arba'een is a public holiday in Iran.
Husayn ibn Ali was the son of Muhammad's daughter Fatima Zahra and the first Imam of Shia Islam Ali ibn Abi Talib.
He became the third Shia Imam in 669, following the death of his brother Hasan ibn Ali.
Husayn remained Imam until 680, when he and his 72 companions were killed in the Battle of Karbala by the army of Yazid I.
Shia Muslims commemorate the Battle of Karabala on the 10th day of Muharram, known as the Day of Ashura.
This day is the beginning of a traditional forty-day period of mourning.
Arba'een is the culmination of the mourning period that commemorates the martyrdom of Imam Husayn.