On Tuesday, habitual conspiracy theorists celebrate what they believe was a 1947 alien spacecraft crash and US cover-up.

Each summer, the world marks Unidentified Flying Objects Day, or UFO Day.

On Tuesday, habitual conspiracy theorists celebrate what they believe was an alien spacecraft crash and a US government cover-up that allegedly occurred in the summer of 1947, in Rosswell, New Mexico.

But what is the celebration? And who started it? Below is all we know about UFO Day.

What is UFO Day?

As defined by the World UFO Day Organization (WUFODO), the World UFO Day is a day dedicated to the existence of Unidentified Flying Objects.

It aims to raise awareness about the “existence of UFOs and with that intelligent beings from outer space,” encouraging people to “think about the possibility of us not being alone in the Universe”.

It also wants to encourage governments to declassify their files on supposed UFO sightings.

How is the day celebrated?

There are various celebrations but according to WUFODO, on this date, people are encouraged to look at the sky and try to identify UFOs.

According to them “t he most important thing is that people collectively open their minds to the subject for one day and send out the message that UFOs are welcome on this earth”.

But was this the first celebration?

Before, this commemoration was celebrated on June 24, the date on which aviator Kenneth Arnold reported seeing nine unusual objects flying over the state of Washington.

Arnold described the UFOs as a “saucer-like” or ” a big flat disk”. This description ended up shaping the way UFOs were imagined.

However, WUFODO established the date on July 2, to lead people to celebrate in large numbers on one day instead of small groups on two separate days.

It commemorates the date on which the famous Roswell incident was exposed to the public.

Rosswell project: the key event

On an unknown day in July,1947, a Project Mogul balloon crashed in the desert near Rosswell, New Mexico.

It was part of a top-secret project by the US Army Air Forces involving flying microphones on weather balloons to extreme heights to detect Soviet nuclear test explosions.

The wreckage of the balloon was first spotted by William Brazel, who described it as a “large area of bright wreckage made up of rubber strips, tinfoil, a rather tough paper and sticks”.

The rapid response of the army to recover the debris and the subsequent cover-up story, which stated that the wreckage was that of a weather balloon, served as the basis for conspiracy theories.

This would pave the way for a craze for extraterrestrial encounters that had a strong impact in 1950s Hollywood, peaking in the 70s and 80s giving rise to classics such as Close Encounters of the Third Kind, ET and of course Star Wars.

What about recent days?

Recently, there has been a wave of alleged UFO sightings by US Navy pilots between summer 2014 and spring 2015, causing the Navy to develop a formal set of guidelines to report them in April 2019.

“The Navy does not think that aliens have been flying in US airspace,” one Navy official told CNN.

But there have been “a number of reports of unauthorised and/or unidentified aircraft entering various military-controlled ranges and designated air space in recent years,” according to the official.

Several navy pilots also told the New York Times about multiple encounters with UFOs with no visible engine or infrared exhaust plumes, but “that could reach 30,000 feet and hypersonic speeds”.

A purported top secret UFO hidden and covered inside a hangar [File: Getty Images]

What do other US presidents have to say about this?

US presidents have been known to talk about the subject of UFOs and the Roswell incident.

Barack Obama during an interview with GQ magazine in 2015 said: “I gotta tell you, it’s a little disappointing. People always ask me about Roswell and the aliens and UFOs, and it turns out the stuff going on that’s top secret isn’t nearly as exciting as you expect.”