In your specific context it's entirely a stylistic choice whether to use the Saxon genitive ('s).
Arguably, Sunday's supper is a slightly atypical example in this area. If it had been Sunday dinner, the apostrophe would rarely be included - but it usually would be in, say, Thursday's supper.
The reason for this is that the noun as adjective (noun adjunct, attributive noun) usage tends to be reserved for contexts where what's being referenced is something known, familiar to the speaker.
Native speakers know that Sunday dinner is a particular kind of meal, so it's actually credible to say something like I sometimes have Sunday dinner on Saturday (it'll be understood to mean a Sunday roast, regardless of the apparent "day mismatch").
But if you said I'm having Thursday supper on Friday this week, people would look at you askew. They'd have to suppose that you know exactly what you (either in general, or specifically in your meal plan for the coming week) have for supper on Thursday(s), but they wouldn't normally know what that meal might be, so it wouldn't make much sense.