Denise explained it very well, but let me elaborate. The expression comes from a line in a Clint Eastwood movie. He meets some criminals in the process of a robbery, and pulls out his gun. The expression is sarcastic, and means "Go ahead, keep doing what you are doing, forcing me to shoot you. If you do that, it will 'make my day', which means it will make my day worthwhile." It is a threat to the criminals, telling them that if they proceed with the robbery, he will be forced to shoot them, which he would like to do anyway, but because he is a 'good guy', he won't shoot them unless he has to.