Why Matam by Shias when maximum mourning period is 4 months?

How do Shias justify their Mourning (Matam) over Battle Of Karbala? We know that maximum mourning period of someone's death is 4 months 10 days (that too for a widow) but they do it every year. Please answer affirming to the fact that Battle of Karbala happened 40 years after Prophet Muhammad's (peace be upon him) death and it has nothing to do with him. *Related question:* How does the Shia school of belief justify Matam (Self-flagellation) and Tatbir (Striking oneself with sword)?