Short answers, pulled from the story.
Cosmonautics Day is an anniversary celebrated in Russia and other post-Soviet states on the 12th of April. It marks the first crewed spaceflight, made on the 12th of April 1961 by Soviet cosmonaut Yuri Gagarin.
Yuri Gagarin was a 27-year-old Soviet cosmonaut who made the first crewed spaceflight on the 12th of April 1961. He orbited the Earth for 1 hour and 48 minutes aboard the Vostok 1 spacecraft.
The Soviet Union established the commemorative day on the 9th of April 1962, one year after Gagarin's flight. In modern Russia it is observed under Article 1.1 of the law "On the Days of Military Glory and the Commemorative Dates in Russia."
The commemoration begins in the city of Korolyov at a statue of Gagarin, then proceeds under police escort to Red Square to visit his grave in the Kremlin Wall Necropolis. Participants continue to Cosmonauts Alley near the Monument to the Conquerors of Space before ending at Novodevichy Cemetery.
On the 7th of April 2011, the United Nations General Assembly adopted a resolution during its 65th session declaring the 12th of April the International Day of Human Space Flight. The United Nations also commemorated the day on the 12th of April 2017 for the 56th anniversary of Gagarin's flight.
The song "Grass by the Home," performed by the band Zemlyane, received official status as the anthem of Russian Cosmonautics in 2009. It succeeded the 1960s song "14 Minutes Until Start" by Oscar Feltsman and Vladimir Voynovich as the day's unofficial anthem.