You can prepare the dish a few hours ahead of time, but not too long or the zucchini will get soft. You can slice the zucchini slices up to a day in advance and store them in the fridge with layers of paper towels between them so they don't release too much moisture from being stacked on top of each other. Store any leftovers in an airtight container, they will last about 2-3 in the fridge, but the zucchini will start getting soft after the first day.