For the sake of convenience, cabbage can be sliced days in advance and kept refrigerated in a securely sealed, heavy-duty plastic bag, but the vegetable will lose vitamin C soon after being sliced or torn. So, buying half of a head (even if it’s well wrapped) is not a great idea. If left whole and wrapped tightly and refrigerated, cabbage will remain fresh and in good condition for two weeks. Since slicing cabbage by hand is not hard to do and makes little mess, it’s better to slice it shortly before cooking. If you don’t need to slice an entire head, only wash the parts that you intend to use right away. Keep the remainder in the refrigerator tightly wrapped with plastic and use within one or two days.