Today (Monday) is the busiest mailing day of the holiday season.

That will make this Thursday the busiest delivery day for letter carriers.

The Post Office expects 16 billion pieces of mail and packages between Thanksgiving and New Year's Eve.

If you absolutely, positively need to have it there before Christmas, Tuesday is the last mailing day for first class mail, though Priority Mail will still be an option through Wednesday, December 21st, and Priority Mail Express until Friday, December 23rd.