Muharram is a month of remembrance that is often considered synonymous with the event of Ashura. Ashura, which literally means the "Tenth" in Arabic, refers to the tenth day of Muharram. It is well-known because of historical significance and mourning for the martyrdom of Hussein ibn Ali, the grandson of Muhammad. Shias start the mourning from the 1st night of Muharram and continue for two months and eight days.
Photos below show 4th day of Muharram in Tehran: