(Probably needs a bit of attention - neither of the suggested English sentences are grammatical or plausible)
How is "She will get ready for Shabbat's dinner." grammatically incorrect?
The problem is not that "Shabbat's dinner" is accepted (though it is 192 vs 378,000 for "Shabbat dinner" on Google, and many of those few instances are the slightly different case of "last Shabbat's dinner" etc). The problem is that the linguistically 'unmarked' option, to which a native speaker will normally lead by the underlying system of options in meaning, is rejected. But we are not here to discuss English :-) The point is simply that the example needs a bit of editing to avoid slowing down the learning of Hebrew.
Okay. That is all fine and good. I'm glad that you were able to make your initial comment clearer.
I translated it with 'saturday dinner', which got rejected. I now that shabbat dinner is usually on friday, but couldn't the saturday dinner be here addressed as well?