You can make the filling in advance and store it covered in the fridge for 2-3 days but I don't recommend prepping the apples a few days in advance as they brown quickly. At best, you can prep them a few hours in advance. To minimize browning, once you core your apples, place them in a bowl of cold water that has been mixed with a pinch of salt. Leave them there for a few minutes, then rinse and pat dry.