The International Workers’ Day, or Laborers’ Day, or simply May Day, is a holiday which for a long time has symbolized revolution and irreconcilable class struggle.  

And Google likewise celebrates May Day with a special homepage. 

This holiday is marked in 142 countries either on May 1 or on the first Monday of May. 

But in some other countries, like the US and Japan, the Labor Day is celebrated on another day.