"What a day" is an English expression used to express surprise, joy, or dissatisfaction with something that happened during the day. The meaning of the expression can be positive or negative depending on the context in which it is used. For example, if the sentence in which it is used is positive, then "What a day" means "This is a good day" or "Today is a great day". On the other hand, if the sentence is negative, then "What a day" means "This is a bad day" or "Today is a terrible day". In general, "What a day" is an expression used to convey strong emotions about something that happened during the day.