That's because it's expected to be the busiest day of the year for the post office.
Seven million customers are expected to visit the post office on Monday alone, so expect long lines when you show up to mail your gifts.
Here are some shipping deadlines to get your packages delivered by Christmas: For first-class mail, the deadline is Dec. 19. For priority mail, it's Dec. 20.
For priority mail express, you've got until Dec. 22.