May Day.

That Day The Guardian offers a history of May Day or rather the history of May Day in the modern era:

"The origin of our present holiday lies in the fight for an eight-hour working day, in which cause the leaders of the socialist Second International called for an international day of protest to be held at the beginning of May 1890. They did so just as the American Federation of Labour was planning its own demonstration on the same date. The UK protest actually took place on a Sunday, and in London alone attracted 300,000 protesters to Hyde Park."