Have a nice day is a fashionable expression used to conclude a conversation or finish a written message in many languages.
The phrase "have a nice day" spread to Britain from the United States. In Britain, the variants "have a fine day" and "have a good day" are frequently used in place of "have a nice day".
For example, in European languages, the standard way to greet someone formally is to wish them a good day. Here are some languages which translate as “Good day”: Romanian: Bună Ziua. French: Bonjour. German: Guten tag.
This is done by hoping the person to whom is addressed experiences a pleasant present. Today is what matters, so we live for the day. The present is more important than the past and the future, so make it your best day.