"What a day" is an English expression that can be translated to Portuguese as "Que dia" or "Que dia incrível" depending on the context. It can be used to express a positive or negative feeling about a day, depending on the tone of the speaker. Here are some examples of how "what a day" can be translated to Portuguese:

• "What a day at the park" - "Que dia no parque"
• "What a beautiful day" - "Que dia lindo"
• "What a lovely day" - "Que dia encantador"

It is important to note that the translation may vary depending on the context and the tone of the speaker. Therefore, it is always recommended to consider the context before translating an expression.