How do you say good day in Japanese?

there is really no term for good day as in good bye but below is 'a good day' 良い一日

Yoi ichi-nichi

Actually, there is, but it's very rarely used. It has a rich girl air to it.

御機嫌よう (ごきげんよう) gokigenyou

It works the same way as "good day" in English, both as a greeting and a farewell.

For normal conversions, you're better off looking up other greetings and farewells.