Holiday cards and packages are being delivered in the next few weeks and some postal deadlines should be kept in mind. The U.S. Postal Service has listed the ship-by dates for delivery by Christmas. First-Class Mail Service needs to be postmarked and in the mailbox by December 20th. Priority Mail Service has a deadline of December 21st. And the Priority Mail Express Service will need to be in the mail by December 23rd. Mail will not be delivered by the U.S. Postal Service on Christmas Day.