CHICAGO (CBS)–If you still have packages or letters to send for the holidays, the U.S. Postal Service suggests getting them into the mail by Tuesday.

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.

About 3 billion of those packages will be delivered this week.

The 2018 holiday shipping deadlines are as follows:

  • Dec. 18 for USPS Priority Mail Express
  • Dec. 20 for first-class mail and packages
  • Dec. 22 for Priority Mail Express