Christmas falls on a Saturday this year. The U.S. Postal Service has published the following deadlines for expected delivery by December 25. (Actual delivery may vary. Some restrictions apply.)

USPS Holiday Shipping Deadlines 2021
https://www.usps.com/holiday/holiday-shipping-dates.htm

Continental U.S.:
December 15: USPS retail ground service
December 17: First-class mail service (including holiday greeting cards)
December 17: First-class packages (up to 15.99 ounces)
December 18: Priority mail service
December 23: Priority mail express service

The USPS also announced that effective Oct. 3, 2021, it slowed the delivery of First Class Mail to focus on increased demand for package delivery. Cross-country mail will now take up to five days. (Mail those holiday cards early!)

Costs to ship packages will also be increasing, temporarily, through the end of the holiday season.