make someone's day make an otherwise ordinary or dull day pleasingly memorable for someone.
☞ The expression is particularly associated with the character 'Dirty Harry' (police detective Harry Callahan) played by Clint Eastwood in the film Sudden Impact (1983) who, when threatened by a criminal, points his gun at him and says 'Go ahead, make my day' (i.e. by giving Harry an excuse to shoot).