https://techcrunch.com/2018/12/13/amazon-extends-free-shipping-until-dec-18-announces-christmas-eve-delivery-in-select-markets/

Amazon will again enable procrastinators to hold off on completing their Christmas shopping until the last minute. The company today announced it will offer free, two-hour delivery up until midnight on Christmas Eve, powered by Prime Now, in select markets. Prime Now is currently available across 30 U.S. cities, Amazon says. In addition, Prime members in over 10,000 cities and towns now have access to free same-day delivery and free one-day shipping up until the final days before Christmas.

Specifically, December 23 is the last day for one-day shipping, while December 24 is the last day for same-day delivery, if orders are placed by 9:30 AM local time.