Imam Hussain who is the prophet Muhammad’s grandson was martyred on the tenth day of Muharram, day of Ashura, in Karbala but why do we have a ten-day of mourning period started from the first day of Muharram?!
Recently, professor Panahiyan, an Iranian cleric who teaches at the universities and Howzah, answered this question as a note to a news channel that I tried to translate the main parts of it:
There might be some different reasons for mourning form the first day of Muharram. It might be because of what Imam Reza, the 8th beloved Imam of ours, said about his father, Imam Kazem (pbuh): “No one could have seen my father’s smile when the month of Muharram came. Sadness was increasing in his face day by day till the tenth of Muharram and when the tenth day of Muharram would dawn, it would be a day of grief and weeping for him.“
Another reason might be that mourners can prepare themselves for the deep grief of Ashura by mourning form the beginning of Muharram because the grave tragedy of Ashura can be more tolerable after one week of mourning. The prophet Muhammad said that “surely , there exists in the hearts of the believers, with respect to the martyrdom of Husayn, a heat that never subsides.” Thus, Every one who feels this heat, knows that entering to the day of Ashura without a previous mourning period is very painful.
For some people a ten-day of mourning period is an opportunity helping them to feel the great tragedy of Ashura.
Actually this mourning period might be a type of volunteering to help not mourning and it seems to us that mourners go into Imam Hussain’s tent from the first day of Muharram in order to help him and not to leave his leader alone. That’s why this period of ten days is like an epic while mourning on Ashura night has just a color of sadness and greif.
 (کَانَ أَبِی ع إِذَا دَخَلَ شَهْرُ الْمُحَرَّمِ… امالی صدوق/128)
 «إِنَّ لِقَتْلِ الْحُسَیْنِ حَرَارَةً فِی قُلُوبِ الْمُؤْمِنِینَ لَا تَبْرُدُ أَبَداً»مستدرک الوسائل/10/318
Drinking water is universally held to be one of the most basic of human rights and is vital to the survival of any living creatures, but Imam Husayn, the Prophet Muhammad’s grandson, was martyred along with all his companions on the day of Ashura in Karbala while they were all thirsty. The tents of his family were pitched just a walking distance from the banks of the river Euphrates and yet he and his camp – including his six month old baby – were not allowed a single drop.
Read more at here
Each year, on the tenth of Muharram, the first month in lunar calendar, Millions of people, dressed in black, take to the streets and mosques to mourn the martyrdom anniversary of the Prophet Muhammad’s grandson, Imam Hussain (PBUH), on a day known as Ashura.
The supreme leader of the Islamic republic of Iran made a speech about Ashura in 2008:
“Why are you disseminating sadness, crying and tears among people?” They complain. This is not sadness and tears for the sake of sadness and tears. Read More…
The day of Ashura falls on the 10th of Muharram, the first month of the Islamic calender.
Every year, in the day of Ashura, plenty of people dressed in black, take to street to mourn the martyrdom of Imam Hussein and his companions.
Here is some photos of Iranian shiite Muslims mourning in the day of Ashura in the center of Tehran, Iran.
Mourning in the day of Ashura in the “Great market of Tehran”, a traditional Market in Tehran, Iran.