A Facebook page encouraging people to storm the so-called Area 51 US Air Force base was removed from the social network at the weekend - but has now been reinstated.
It was removed by "mistake", a spokeswoman for Facebook said.
The page's creator had received a message on Saturday saying his event broke Facebook's community standards.
Two million people have indicated they will attend, but the US Air Force has warned people not to go near the base.
"What happened with the event? It disappeared a couple [of] days ago and is now back," wrote one Facebook user on the reinstated Area 51 page.
Area 51 has long been the focus of conspiracy theories that allege the base conceals information on alien life and technology.
While there is no proof for this, there are many fans of the theory and this has made Area 51 an iconic location in alien-themed popular culture.
The Facebook page has been flooded with jokes and memes about aliens since its creation.
But jokes about going through with the plan to "storm" the base have already drawn a frosty response from the US military.
"[Area 51] is an open training range for the US Air Force, and we would discourage anyone from trying to come into the area where we train American armed forces," a spokeswoman for the US Air Force told the Washington Post in July.
The real Area 51 is officially known as Homey Airport and is north of Las Vegas in the state of Nevada.
It is closed to the public and patrolled by armed guards and flying into airspace above the base is restricted.