The button is hidden when the date contains subscription meals (use the subscription page instead) or when every item on that date is a type that doesn't support date changes on the order page — for example extras-only, gift cards, custom charges, grab & go, POS menu, catering, one-time fees, labor & equipment, real time (Uber) deliveries, venues, or products. When the calendar is empty, none of the next 60 delivery days (starting tomorrow) has all of the meals in that date on its menu — the picker only enables dates where every meal is available. Add the meals to a future menu, or refund and re-order.