Ashura marks the culmination of annual remembrance of the tragic 7th century battle in Karbala. As per the Islamic calendar, the day is observed on tenth date of Muharram - the first month of Hijri year. The gregorian date varies by a single day in region to region as it depends upon sighting of the moon (at start of Muharram). The day remains a major observance for Shia and several sub-sects of Sunni Muslims who recall the martyrdom of Imam Hussain (AS).