9/11 is also known as Patriot Day. Above, the 'Tribute in Light' is reflected on the 9/11 Memorial on Sep. 11, 2014 in New York City. Eric Thayer/Getty Images

Often referred to colloquially as 9/11, Patriot Day is marked every year on September 11, in memory of the nearly 3,000 people who died in the attacks of Sept. 11, 2001. Because it's not a federal holiday, work and school proceed as usual, even though many Americans make a point of remembering the day on their own, or with family, friends and colleagues. In memory of 9/11, here are some facts, events and activities for the observance of Patriot Day.

Flags at half-mast: The president typically issues a proclamation declaring that American flags should be lowered to half-staff out of respect for the people who died as a result of the attacks, when hijackers flew two planes into the Twin Towers of the World Trade Center and another into the Pentagon in Arlington, Virginia. Passengers on a fourth plane managed to wrest control of the craft, crashing it instead into a field in western Pennsylvania.

Moments of silence: Many people and communities choose to hold a moment of silence at 8:46 a.m., when the first plane flew into the World Trade Center.

Special services: Churches, other houses of worship and community centers often hold special services, prayer groups or other meetings in commemoration. The Pentagon Memorial, for example, is holding special services for families of the victims.

Visiting memorials: Those who lost loved ones may visit memorials for those who died, and often decorate them with flowers. Note that the 9/11 Memorial and Museum, located at the World Trade Center in New York City, is closed during the day to the public for a special commemoration ceremony Friday.

Tribute in Lights: Although it is closed for most of the day, the 9/11 Memorial and Museum re-opens at 3 p.m. Friday to the public for a special viewing of the Tribute in Light, where two beams of light, in the shape and spacing of the Twin Towers, are shot four miles into the sky, visible from up to 60 miles away.

National Day of Service: Launched in 2002, the National Day of Service asks people to do at least one good deed on 9/11, in honor of those who died that day. Numerous community service organizations and charities offer opportunities for people to give back, paying tribute to the lives lost on Sept. 11, 2001.

Good to know: Don't confuse 9/11 with Patriots' Day, which is celebrated on the third Monday in April, generally in Massachusetts. Patriots' Day commemorates the Battles of Lexington and Concord, which took place in 1775 and marked the start of the Revolutionary War.