What's the difference between having fun and having a good day?

1, and have a good day all mean "have fun, have fun", and their meanings are basically similar.

2, but strictly speaking, the difference between the two should be:

①have a good time generally means "have fun (for a while)", for example

I wish you a happy summer vacation! I wish you a happy summer vacation!

Have fun at the weekend!

②have a good day means a relatively short period of time, including only the day, that is, "I wish you a happy day".

This is the difference in meaning between the two.