Question: Why can you say Kiddush before nightfall if you made an early Shabbos but you have to at least have part of your Shabbos meal once it gets dark in order to fulfill the mitzvah of eating your first Shabbos meal?
A. There is actually an argument amongst the Achronim whether one can eat the meal before dark. The Machatzis Hashekel explains the view of why you have to wait till dark which is because we learn the mitzvah of having 3 meals on Shabbos from the three times it says “hayom “ in the verses that discuss Shabbos, and only Shabbos itself is called “Yom Hashabbos” but what’s rabbinically added onto Shabbos is not. Why doesn’t Kiddush have the same parameters?
Answer: Saying Kiddush by itself signifies the sanctity of the day so it can be said before nightfall but it’s not apparent that you are eating the meal for Shabbos until the day comes about at night fall therefore at least part of the meal should be eaten at nightfall to recognize that the meal is being eaten for Shabbos.