Muharram is the first month of the Islamic calendar. On the first day of Muharram, the Islamic New Year is observed by Muslims. The Islamic calendar is a lunar calendar, and is 11 to 12 days shorter than the solar year. Hence it is a little different from the Gregorian calender that is used in the western nations. When compared with the Gregorian calendar, which is a solar calendar, the lunar month of Muharram shifts from year to year.
The month of Muharram is of great religious significance to Islamic people the world over. It is held to be the most sacred of all the months, excluding Ramadan. The word "Muharram" is often considered synonymous with "Ashura", the tenth day of the Muharram month.

10th of Muharram (the day of Ashura / Ashoora) is observed as an important day by both Sunni and Shia Muslims – however, fordifferent reasons.

Most scholars believe that Ahsura is named as such because of “tenth” of Muharram (ten is translated as “Ashara” in the Arabic language). Sunni Muslims look at Ashura as a day of “respect and gratitude” (for Prophet Moosa and his nation), while Shia Muslims believe that day to be a day of mourning and sorrow.

Sunni Muslims

Based on the Hadith of Prophet Muhammad (saws), Sunni Muslims celebrate Ashura as the day when Prophet Moses (Moosa) fasted on that day because Allah saved the Israelites from their enemy in Egypt. 

Shia Muslims Observance

Shia Muslims observance of Ashura is different altogether. They observe Ashura as the day of martyrdom of Hussein ibn Ali, the grandson of Prophet Muhammad at the Battle of Karbala. Shia Muslims, therefore, consider this a day of sorrow and observe it as such by refraining from music, listening to sorrowful poetic recitations, wearing mourning attire, and refraining from all joyous events (e.g. weddings) that in anyway distract them from the sorrowful remembrance of that day.