The typical phrase is "You made my day" or "You've made my day." In either case, it means that someone has done or said something that made you very happy, and it made the day more special for you.
Your alarm clock didn't wake you up and your hair dryer broke while you were using it. You arrived at work late with your hair in a mess. But a co-worker walks by and asks you, "Have you been on a diet? You look like you've lost weight."
You smile and respond, "Thanks. You've just made my day."