I have seen that some people say "What a day!" sometimes. And it appeared in many stories, movies,etc... So I wonder " What's this idiom mean?". Can you help me explain it? Thanks a lot ^^
It all depends on context. If somone has had a terrible day full of disasters and disappointments they might say "What a day!" meaning "What an awful day/That was a terrible day".
If they had a fantastic day of joy and happiness and lovely things, they might say exactly the same thing but they would mean "I've had a great day".
If you don't know what kind of day someone had and they exclaimed "What a day!" to you, you would have to get them to clarify by asking something like "Why? What happened?" Once they've told you, you would know what they meant.
We use "What a ..." quite a lot. An unpleasant person might cause the comment "What a b*stard!", a stupid person "What an idiot!", a very handsome man "What a hunk!"
It's not really an idiom as such.
