"Quel est le plat du jour ?"
Translation:What is today's special?
If you're allowing this:
What is the dish of the day?
Then you have to allow this:
What is the meal of the day?
A real world example:
Because "dish of the day," and "meal of the day," are set expressions in English. "Plate of the day," sounds more like a celebration of a piece of ceramic than the meal placed upon it. Indeed, I can imagine such a sign at the Wedgwood Visitor Centre.
I suspect it is the word 'menu', which means the complete list of offerings. «Plat» is a specific offering. I.e., "The restaurant has fifty items on its menu («carte»), my favorite dish («plat») is the mussels.
It is possible that a restaurant could have a special 'menu of the day' (some very high end restaurants change their entire menu daily, etc). In those cases, the French sentence would have to be different, something along the lines of "Nous avons un menu du jour".