A Women Looks Out Over Some Of The 2,977 Flags Laid Out To Signify The People Who Lost Their Lives In The 9/11 Attacks In New York, Washington, D.C., And Shanksville, Pennsylvania, During A Remembrance Event On The Campus Of Cuyamaca College In El Cajon,
A women looks out over some of the 2,977 flags laid out to signify the people who lost their lives in the 9/11 attacks in New York, Washington, D.C., and Shanksville, Pennsylvania, during a remembrance event on the campus of Cuyamaca College in El Cajon, California September 11, 2014. Today is the thirteenth anniversary of the 9/11 attacks on New York, Washington, and Pennsylvania. REUTERS/Mike Blake

Sept. 11, 2014 marks the 13th anniversary of the horrific terrorist attack on the U.S. On Sept. 11, 2001, al-Qaeda terrorists attacked the two pivotal buildings of the U.S. and left thousands dead and injured. The day witnessed massive loss of life and property and engraved painful memories in the hearts of millions.

The 9/11 attacks are referred to as the series of four terrorists attacks that rocked America more than a decade ago. The attack was executed by 19 militants using four hijacked airliners. Two aircrafts hit the south and north towers of the World Trade Center in Manhattan and a third one bumped off in the Pentagon building in Washington D.C. The fourth airliner reportedly fell in remote area of Pennsylvania. These attacks occurred between 8:46 a.m. and 9:37 a.m. and took the life of almost 3,000 people.

Each year, the day is observed as Patriot Day or National Day of Service and Remembrance. Several vigils and memorial services are held and moments of silence are observed. This year the National September 11 Memorial Museum was opened.

On Sept. 11, 2014, U.S. President Barack Obama, first lady Michelle Obama, along with Vice President Joe Biden pay respect to the victims and remembered the terrible day by observing a moment of silence at the White House.

The president also took to the social media website Twitter to pay tribute and promises to never forget the catastrophe. Hashtags #NeverForget and #NeverForgetSeptember11 started trending when Hollywood stars promised to do the same. Celebrities like Taylor Swift, Gigi Hadid, Lindsay Lohan, Julianne Hough, Ellen DeGeneres, Andy Cohen and Carson Daly tweeted messages remembering the day that is hard to forget for everyone. Scroll down to read the 9/11 Twitter posts by celebrities.

Meanwhile, there were celebrities who participated in charity events to raise money for the families of those who suffered the loss during the Sept. 11 terrorists attack. According to New York Post's Page Six, Hollywood celebrities Pamela Anderson, rapper 50 Cent, Jenny McCarthy, Edie Falco, Uzo Aduba and sports star Carmelo Anthony gathered at Cantor Fitzgerald and BGC to take part in the event honouring the sufferers of 9/11 attacks. More than 650 employees of the companies lost their lives in the 9/11 attacks.

"Thirteen years after small and hateful minds conspired to break us, America stands tall." —President Obama #NeverForget

— Barack Obama (@BarackObama) September 11, 2014

NEVER FORGET 9/11/01. To all the brave hearts that were there for others on that day, we can never repay you, but we will always remember. — Gigi Hadid (@GiGiHadid) September 11, 2014

Visiting the @Sept11Memorial this year was something I'll never forget, in memory of a day we will always remember with tears in our eyes.

— Taylor Swift (@taylorswift13) September 11, 2014

Never Forget #9/11 http://t.co/B3CYSDivSq — Julianne Hough (@juliannehough) September 11, 2014

Send a little extra love into the world today, and remember why we need it. #NeverForget #911Anniversary

— Ellen DeGeneres (@TheEllenShow) September 11, 2014