Ashura, the 10th day of Muharram is observed differently by Muslims depending upon which branch of Islam one follows. For Shia Muslims, Ashura is mainly a day of mourning and remembrance of the martyrdom of Imam Hussain and his family and companions at the Battle of Karbala. 
Calendars are a crucial foundation of society. They help us keep track of historical knowledge and planning things for the future. Throughout the history of the world, humans have created many calendars such as the Julian and the Gregorian Calendar.
Muharram is the first month on the Islamic calendar and is a word that literally means sacred. In the Holy Quran the four sacred months are a time when warfare is prohibited.
“The year is twelve months, of which four are sacred: three consecutive months,
Muharram is the real new beginning. Each year, Muslims around the world experience the first of the month on a different day, in a different manner, and eventually in different seasons.
These guidelines was adopted in the Islamic Shura Council of North America meeting in Chicago moon sighting on January 19th 1995. It was signed by all the leaders.
In AD 638, six years after the death of the Prophet Muhammad, Islam's second caliph Umar recognized the necessity of a calendar known as Hijri calender, to govern the affairs of the Muslims. This was a practical matter.