World Contact Day was first declared in March 1953 by an organization called the International Flying Saucer Bureau (IFSB), as a day on which all IFSB members would attempt to send a telepathic message into space.
The IFSB voted to hold such a day in 1953, theorising that if both telepathy and alien life were real, a large number of people focussing on an identical piece of text may be able to transmit the message through space.
[1] IFSB members focused on the following message during 1953: Calling occupants of interplanetary craft!
[2]The 1953 celebration is referenced in the song "Calling Occupants of Interplanetary Craft", created in 1976 by Klaatu and later covered by The Carpenters.
[3] On the event's 60th anniversary in 2013, World Contact Day was extended to a whole week.