The act of a person who mourns; sorrowing or lamentation.

Mourning is the period or interval during which a person grieves or formally expresses grief, as by wearing black garments.
adjective

To feel or express sorrow or grief over (misfortune, loss, or anything regretted); deplore. To grieve or lament over (the dead). To utter in a sorrowful manner, sometimes those who are grieving the dead loose their apatite for food, and thus it becomes a type of fasting which is activated through the state of mourning.

Mourning is, in the simplest sense, grief over someone’s death. The word is also used to describe a cultural complex of behaviours in which the bereaved participate or are expected to participate. 

Customs vary between cultures and evolve over time, though many core behaviors remain constant.

Wearing black clothes is one practice followed in many countries, though other forms of dress are seen. Those most affected by the loss of a loved one often observe a period of grieving, marked by withdrawal from social events and events which can give pleasure to one’s soul, so that the person may give more time to the grieving his/her dead.

People may follow religious traditions for such occasions. According to the hebrew Jewish calendar the 3 weeks of mourning commemorates the period which falls on the 17th of Tamuz to the 9th of Tisha B’ Av.

Mourning may apply to the death of, or anniversary of the death of, an important individual like a local leader, monarch, religious figure, etc. State mourning may occur on such an occasion. 

Article written by Conscious lifestyle/part taken from Wikipedia.org