They start the moment you check in and they are not each day specific. you have a total number of credits for the number of nights and people you have they could all be used the first day if you wanted to , of course no one really could eat that much LOL the total of them expire on Midnight of the day you check out, so if you have any left that last day you can use them any time until midnight of that day.
That is the only time midnight enters the equation.