Mourning is a time of sadness because of a loss. When you're in mourning after a loved one dies, it is good to lean on your friends who understand why you are so sad.
Mourning is an expression of grief or a time of grieving that follows a loved one's death or other serious loss. Depression, confusion, even losing interest in friends and social activities all can occur when someone is in mourning. An old-fashioned meaning of mourning is specifically the black clothing people traditionally wear following a death. Don't confuse mourning with morning, the early part of the day.