The U.S. Postal Service has announced its shipping deadlines for packages to arrive by Christmas. December 16 is the cutoff for 5-day shipping using USPS Ground Advantage and First-Class Mail services. You have until December 18 to mail parcels if you pay for four-day-shipping. You have until December 21 if they use the USPS's Priority Mail Express one-day shipping option. These dates exclude Hawaii and Alaska. The last mail day before Christmas is Saturday December 23rd.