Short answers, pulled from the story.
The Soviet Union established Cosmonautics Day on the 9th of April 1962. This decision transformed a single historical event into an annual national observance exactly one year after Yuri Gagarin's flight.
Yuri Gagarin orbited the Earth for 1 hour and 48 minutes aboard the Vostok 1 spacecraft on the 12th of April 1961. He was only 27 years old when he completed this historic mission.
Modern commemoration ceremonies begin in the city of Korolyov near Gagarin's statue. Participants then proceed under police escort to Red Square for a visit to Gagarin's grave in the Kremlin Wall Necropolis before continuing to Cosmonauts Alley.
The United Nations General Assembly adopted a resolution on the 7th of April 2011 declaring the 12th of April as the International Day of Human Space Flight. The 65th session of the assembly passed this measure to dedicate the date globally.
The hit Grass by the Home performed by the Russian VIA band Zemlyane received official status as the anthem of Russian Cosmonautics in 2009. Russian cosmonauts have traditionally taken this song with them when assigned for orbital deployments.