It's best to prepare it within a couple of hours of serving it. This helps keep the flavors of the foods separated, especially cheeses with that have strong flavors. Plus, it keeps the bread and crackers fresh. You can prepare the meats and cheeses in advance and keep them in an airtight container or freezer bag in the fridge. You can also add the jams and other wet ingredients like pickles or olives to the small bowls, covered in plastic wrap in the fridge. Then all you have to do is quickly assemble it when you're ready to serve it.