September 11, 2001 changed every American’s life. It was a day that no one in the country has forgotten, and for 12 years people have thanked the heroes of 9/11 and mourned its victims. This Wednesday was no different. Hollywood celebrities like Miley Cyrus, Lady Gaga and P. Diddy all took to the Twitter accounts to express their condolences for the lives lost.

On Twitter, which is notorious for being a bullying social media site, trending topics included “where were you,” “always remember,” “R.I.P.,” “Pentagon,” “never forget” and “World Trade Center,” on the anniversary of the worst attack on American soil since Pearl Harbor.

There are millions of heavy hearts on every Sept. 11, but it’s also a reminder how the country united as one in the face of terrorism when two hijacked jetliners crashed into the World Trade Center, one into the Pentagon and one into a Pennsylvania field. "Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation," President Barack Obama said in a memorial service at the Pentagon.

As Reuters reported, hundreds of people gathered at the National September 11 Memorial Plaza, where the towers stood, on Wednesday morning as the victims’ names were read aloud from the 1993 and 2001 terror attacks. Many of the readers addressed their loved ones.

Eleanor Salter said of her daughter Catherine Patricia: "Cathy, your brothers and sisters still miss you. All the people whose lives you touched," Salter said. "As for me, life goes on. But it would have been really fun with you around. I miss your smile.”

Countless pictures and tributes were posted on Twitter on September 11’s 12th anniversary, and some of the Hollywood's A-listers also joined in the remembrance: