"We honor all those who risked and gave their lives in the minutes, hours, months and years afterwards," he said.
"But it's so hard. Whether it's the first year or the 20th," Biden added. "No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago."
Nearly 3,000 people were killed, including more than 2,600 at the World Trade Center in New York, after hijackers seized control of airplanes and used them to attack the World Trade Center's twin towers and the Pentagon just outside Washington.
On Saturday (Sept. 11), Biden and first lady Jill Biden will visit New York City and the Pentagon in Arlington, Virginia, as well as Shanksville, Pennsylvania, where passengers forced down an aircraft believed to have been headed to the U.S. Capitol or White House.
Biden said the central lesson for Americans to take away from September 11 was that "unity is our greatest strength."
"We learned that unity is the one thing that must never break," he said.