Many Americans may no longer depend on the post office for the daily delivery of important documents. But when Christmas rolls around, the nation's postal workers are as essential to the holiday as Santa and his elves.

A pop-up holiday post office in Washington, D.C., 1922. | (Science History Images / Alamy Stock Photo)

This holiday season, the U.S. Postal Service expects to deliver 15 billion pieces of mail, including some 850 million packages. During its busiest week of the year, Dec. 18 to 24, the department will process and transport an astounding three billion items.

"We plan all year for this," Sue Brennan, a USPS spokeswoman, told The New York Times. "This is something that we've done for 240 years-ish, so we're pretty good at doing it."

In addition to delivering the heaping piles of packages and Christmas cards, American postal workers have another very important holiday task: answering letters to Santa.

Children line up to mail their letters to Santa Claus at special mailboxes in Downey, California, 1947. | (Underwood Archives/UIG/REX/Shutterstock)

Children have relied on the post for delivering their handwritten notes to the North Pole since at least the 1870s. Technically, the policy was to designate such mail as "undeliverable" and send it to the Dead Letter Office to be destroyed. But some postal workers took it upon themselves to read and respond to the neediest of children. Clerks collected money or solicited donations from businesses to get the kids the presents they yearned for; others turned the letters over to charities or wealthy individuals who could engineer a few Christmas miracles.

In 1913, U.S. Postmaster General Frank Hitchcock sanctioned local postmasters to respond to Santa letters or distribute them to philanthropic organizations to be fulfilled — a program that became known as "Operation Santa." This year, New York City is piloting the first digital Operation Santa program so that local residents can "adopt" letters and fulfill wishes online.

In honor of America's intrepid postal elves, here's a look at the endearing chaos of delivering Christmas in decades past:

Circa 1910-1915. | (Library of Congress/Flickr)

(Underwood Archives/UIG/REX/Shutterstock)

Christmas mail being sorted in New York City in November 1918. | (American Photo Archive / Alamy Stock Photo)

A post office worker dressed as Santa Claus on Dec. 14, 1966. | (Trinity Mirror / Mirrorpix / Alamy Stock Photo)

In 1924, New York's December mail volume was so high the post office rented extra space in the Brooklyn Armory. | (Courtesy Smithsonian National Postal Museum)

Sorting holiday packages during WWII. | (Public Domain/Courtesy Smithsonian National Postal Museum)

Carriers set out from the General Post Office in New York City to deliver mail in 1955. | (Vecchio/Three Lions/Getty Images)

Mail pours down a chute at the General Post Office in New York City near Christmas in 1955. | (Vecchio/Three Lions/Getty Images)

People queuing along Wall Street in New York to mail their Christmas cards and packages at a pop-up post office. | (Sherman/Getty Images)

Circa 1950s. | (Smithsonian National Postal Museum)