The word "good day" is used to say greeting or goodbye during the day. A good day would be how you would say farewell to a friend with whom you had just had lunch. He or she would reply, "Have a good day!" In English usage, there are two ways of expressing the idea of "a good day": one is to use the word "good," the other is to use "well." For example, you can say either "Have a good day!" or "Have a well day!" Both forms are acceptable and mean the same thing. In fact, they are interchangeable. It is important to note that although both forms are used to say "have a good day!", "have a well day!" has more of a formal feel to it. You would use this form when saying goodbye to someone, for example. You can also say "Have a nice day!" which means the same as "Have a good day!" but it also implies that you want something to eat or drink at some point in the future. For example, if you were to say "I will see you tomorrow," then this would be correct because seeing is not going to happen today.