Translation:Bon dimanche !
I don't think I've ever heard a native English speaker say the phrase "Good Sunday", or "Good <any day of the week>"
This is French though. I hear it every Sunday I go to Carrefour shopping.
It would be a "happy" day of the week unless you were actively wishing them to have a good performance on that day, when it would change to "good day of the week" or "good day on day of the week". The comments box is stripping out my brackets around day of the week
These are a little too easy. They could up the ante a bit. Add bonne.
Un physicien moderne étudie la physique quantique les lundis, mercredis et vendredis et médite sur la théorie de la relativité gravitationnelle les mardis, jeudis et samedis. Le dimanche, il prie... pour que quelqu'un trouve la corrélation entre les - Norbert Wiener