Why is Muharram called a Mourning festival? What is its significance?

Muḥarram is the first month of the Islamic calendar. It is considered one of the four sacred months of the year and held to be the second holiest month after Ramadan. It is also referred to as Al-Hijra. Since Muslims follow Islamic calendar which is a lunar calendar (based on the cycle of Moon), their festivals move from year to year. This Year Muharram falls on 21st September 2018.

The tenth day of the Muharram month is known as Ashura. It commemorates the death of Imam Hussein and his family.  That is a day of mourning and fasting for Shia Muslims. It is believed that Prophet Muhammad (R.S.A.) asked Muslims to fast on this day and one day prior to that, that is the day of Tasua. 

Muslims on this day pray for the martyrs and refrain from all joyous events. They spend that day remembering the murder of Hussein ibn Ali who was the grandson of Prophet Muhammad (R.S.A), and his 72 followers. The mourning for some sects of Muslims begin from the first night of Muharram and some start from 7th day onwards and continues till the day of Ashura.