CHICAGO (CBS)—The U.S. postal service says today is the last day to send first-class mail for it to arrive by Christmas.  

That deadline includes greeting cards and first-class packages.

The deadline for Priority Mail Express packages Dec. 22.

The week of Dec. 17-21 marks the busiest period of the year for the post office, which expects to mail an estimated 16 billion pieces of mail between Thanksgiving and New Year’s Day, USPS says.