It's short for "Make my day wonderful / complete / the best ever" It's not always positive. It is said sarcastically at times. "I'm afraid I have bad news." "Sure, make my day!" It was made especially famous by on of the "Dirty Harry" films, starring Clint Eastwood. He would have a gun on a crook, who was in reach of a gun himself and start reaching for it. At this point Eastwood would say, "Go ahead, make my day." The meaning was that if the crook actually tried to get the gun, it would give Eastwood a chance to kill him and make Eastwood's day a good one.

To make someone happy.

you made my day= meaning made me extremely happy

Happy day Make my day

Happy day Make my day