More than 21 million Shia Muslims took part in the Arbaeen pilgrimage in Iraq this year, which reached its peak on Sunday with attendees displaying their support for Gaza. Arbaeen, which means forty in Arabic, marks the 40th day of mourning for the martyrdom of Imam Hussein, the grandson of the Prophet Mohammed and a founding figure in Shia Islam. It is one of the world’s biggest religious gatherings and a major event for Shia Muslims, who are the majority in Iraq and Iran. Karbala, where Hussein and his brother Abbas are buried in two enormous mausoleums facing each other, is the centre of th…