That's the same Wiki reference I used.
1891 predates 1917. And the Haymarket Riots which it commemorated were in 1886. The Bolshies no doubt expropriated a celebration that already existed under the Czars.
Do you actually feel more comfortable celebrating the Day of the Worker in September? Or would you prefer it not be observed at all? Or maybe you are just jealous no one anywhere has parties, parades and barbecues on the International Day of The Capitalist.