Cosmonautics Day is a holiday celebrated (mainly in Russia) every April 12 to commemorate the first manned earth orbit. It was established in the USSR on April 9, 1962.
It is celebrated in honour of the historic first manned space flight made on April 12, 1961 by 27 year old Russian cosmonaut Yuri Alekseyevich Gagarin of the USSR, who made one complete orbit around the Earth (lasting 1 h 48 min) aboard the spacecraft Vostok 1.
Soviet stamps dedicated to Cosmonautics Day from my collection:
KINGDOM OF TONGA COVER - Thanks Xu Wei