They commemorate the martyrdom of Hussain as a religious occasion called Muharram.
The occasion starts on the 1st day of Muharram and lasts for 10 days until 10th of Muharram.
As Muharram approaches, they put on black clothes, as black is regarded as a color of mourning. During the entire 10 day period, they keep themselves away from music and all joyous events that can distract them in anyway from the sorrowful remembrance of that day.