The 10-day Ashura rituals are to mourn the killing of the Prophet Muhammad's grandson, Hussein, by armies of the caliph Yazid in the year 680.
Ashura, which means tenth in Arabic, falls on the tenth day of the Muslim month of Muharram. The festival's climax this year falls on Sunday.
Last year, some 7 million Shia pilgrims, both Iraqi and foreign, travelled to Karbala during Ashura under the protection of 20,000 soldiers and police. Neverthess, some 263 died in the southern cities of Basra and Nasiriya.
In 2004, the first Shia pilgrimage to the Iraqi city of Karbala following the removal of the late Saddam Hussein's government was marred by attacks which also killed and wounded hundreds.
In a third attack in the northern Iraqi city of Mosul, two people were killed and five others were wounded near the Syrian Orthodox church of St Thomas.
The church's priest, Abdul Massih Dalmay, said that Christians in Mosul were being targeted at Christmas approaches. "The security forces, police and army must provide security. The army are responsible for this".
One witness to the attack said the bomb was hidden in a handcart used to carry flour that was then wheeled toward the church.
The attack was the sixth on Christians in Mosul in less than a month.