The “Glory to the first cosmonaut Y.A Gagarin!” poster was in celebration of the Soviet Union sending the first man into outer space and to orbit the planet earth. This was published near the beginning of the Soviet v’s U.S.A space race which ran from 1957 to 1969.
Following Gagarin’s successful return from space he became a an international celebrity and was awarded the Hero of Soviet Union. Unfortunately Gagarin died in 1968 as a result of a plane crash.
This poster is part of a series of soviet space posters that celebrates this great achievement. Another poster, that mocks the fact that the U.S.A had not yet achieved this feat is the Het! Soviet Space Poster.