Delivery is not a single event. Once the window opens, any short holding a position may issue a delivery-notice on any business day, and the clearing house assigns it to a long. The window usually runs a few weeks and ends several days after the last-trading-day.
Because the short chooses the day, the long carries timing risk: capital must be available, and storage or transport arranged, at any point in the window. This optionality is part of why deliverable futures trade slightly cheap to their theoretical value.
Example: a December gold contract with first notice on 30 November, last trading day 29 December and last delivery day 31 December gives shorts a 22-business-day window in which to tender 100-ounce bars against each contract.
Related: delivery-notice, first-notice-day, last-notice-day, stopping-delivery, physical-delivery