Many restaurants offer a menu which doesn't change from day to day, and in addition offer one choice which varies from day to day, perhaps depending on which ingredients are available. This choice can be called dish of the day (perhaps “soup of the day”, etc.) or special (“today's special”) or perhaps other expressions (what else is there?).
How prevalent are the various ways of formulating this concept? Does it vary between dialects? Is there a nuance in meaning?
(I guess really fancy restaurants would use du jour…)