This is What's Trending Today.

Sunday is September 11. It is the 15th anniversary of the day thousands of people were killed in a terrorist attack in the United States.

Early that morning, two airplanes crashed into the towers of the World Trade Center in New York City. All the people on the planes were killed, along with thousands of people in the buildings.

Another airplane crashed into the Pentagon building in Arlington, Virginia, outside of Washington, D.C. The Pentagon is the office of the U.S. Department of Defense.

A fourth airplane crashed in a rural part of Pennsylvania, when passengers fought back against terrorists who were planning to crash it into a Washington, D.C. landmark.

On social media, people are posting messages about their memories of that day, using the hashtags #NeverForget and #September11.

Ben Sturner lives in New York City. On Thursday, he posted a photo to Twitter showing the reflection of light off one of the new towers at the old site of the World Trade Center. It was shared and liked thousands of times.

Staten Island is a small part of New York City. It is an island in New York harbor, close to the Statue of Liberty and the location of the World Trade Center. Many people in that small community died.

The local newspaper is remembering their lives by tweeting their photos one at a time.

On Friday morning, many U.S. lawmakers gathered outside the Capitol Building in Washington, D.C. to remember those who were killed. They participated in a moment of silence and also sang "God Bless America."

Some people will run almost 400 kilometers from the Pentagon to New York City to honor the firefighters, policemen and rescue workers who saved many lives that day. They started on Friday in Virginia and will finish on Sunday in New York City.

But the anniversary of September 11, 2001, would not be complete without some businesses promoting sales. In the U.S., that is a big mistake.

Many people used Twitter and Facebook to complain about a small mattress store in Texas that made a commercial related to September 11.

The video featured a woman standing in front of two stacks of twin-sized mattresses. The World Trade Center buildings were also called the Twin Towers.

And that's What's Trending Today.

I'm Dan Friedell.

Dan Friedell wrote this story for Learning English. Kelly Jean Kelly was the editor.