The usual way is Skip This Day with Create new day? ticked on an existing open day, which appends a new date after the last one. For a specific date, open the subscription with that date selected (?dd=YYYY-MM-DD) and use the create-date action with the customer's plan order number (&action=create_date&ckid=<order>): the date is created with the subscription's days/meals/portion/categories, provided the subscription delivers on that weekday ("No deliveries for this day"), the order belongs to this subscription ("This checkouts is not from this subscription") and it is a plan order ("No subscription on this checkout"). In the current version this action has no button on the page, so it is normally used by support.