The busiest mailing day for holiday cards/packages is December 17th, when more than 655 million pieces of mail are expected to be processed " compared to 538 on an average day. The busiest delivery day for letters will be December 19th and the busiest day for packages will be December 20th. USPS officials expect that 18 billion cards, letters and packages will be delivered between Thanksgiving and New Year TMs Eve.To make sure you have access to your packages, post offices will be open on Christmas and New Year's Eve, but some locations will have shortened retail lobby hours. Mail delivery will be delivered as normal.In observance of the holidays, Post Offices will be closed December 25th and January 1st. Post Offices will resume regular business hours on December 26th and January 2nd.