Last-minute Santas are heading to the post office in an effort to get their cards and packages to their destinations before Christmas.

Tuesday was the busiest day of the year for the mailing of cards, according to the U.S. Postal Service. Monday was the Postal Service’s busiest day for delivery of packages.

Overall, the Postal Service expects to deliver 16.5 billion letters, packages and cards between Thanksgiving and New Year’s Eve.

It’s still not too late to get packages and envelopes under the tree before Christmas. But you’ll need to hurry:

• The Postal Service guarantees delivery by Christmas Day for Express Mail accepted by Thursday to be delivered to designated ZIP codes. Also, Priority Mail packages shipped by today will arrive in time for Christmas.

• UPS says packages shipped as late as Friday using next-day air with Saturday delivery will arrive at their destinations by Christmas Eve. Items shipped today or Thursday by second-day air also will arrive in time. Thursday is expected to be UPS’ busiest day of the year.

• FedEx also has several shipping plans that offer delivery by Saturday for shipments sent as late as Friday. FedEx said its busiest day for shipping was Dec. 12.