Fourty days before Easter and the beginning of Lent when all the Catholics walk around with smudges on their foreheads and begin a fast from some vice or food or luxury and go around complaining to their friends about how great a sacrifice they are making when they are asked about the black smudge on their forehead.

It is supposed to be for the purpose of remembering Christ's sacrifice on the cross. So that everytime you crave the thing you are giving up you will remember the crucifixion and be thankfull or do something good for people.