I think that nobody can deny that Karl Marx is a figure who shaped history in one way or the other.
Not speaking about him would come close to denying history.
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So why not celebrate Hitler’s birthday?
Socialism/Communism is still influential.
Socialist/Communist regimes still govern parts of the world even with the disappearance of the Soviet Union/Eastern Bloc.
Nazism is completely dead as an ideology. That’s why Karl Marx is the political thinker of the 19th Century.
He is still followed by hundreds of millions of people around the world.