The explanation of "May 1st International Labor Day" in the Chinese-English dictionary (source: Baidu Dictionary):
1. May 1; the International Labor Day; May Day
Also called "May Day", it is fixed on May 1st every year. It is a festival shared by the working class and laboring people all over the world.　
　
　　The May 1st International Labor Day originated from the workers' strike in Chicago, USA. On May 1, 1886, more than 216,000 workers in Chicago went on strike to demand an eight-hour workday. After a hard-fought bloody struggle, they finally won. To commemorate this great workers' movement, in July 1889, the Second International declared that May 1st of each year would be set as the International Labor Day. This decision was immediately responded to by workers around the world. On May 1, 1890, workers from European and American countries first took to the streets, holding grand demonstrations and gatherings to fight for their legitimate rights and interests. From then on, every year on this day, laborers around the world hold meetings and parades to celebrate.