What is May Day? Why do thousands mobilize on Seattle streets on May 1 each year? Here's how May Day became a day for labor rights. Author: Taylor Mirfendereski Updated: 1:15 PM CDT May 1, 2021 Why do thousands mobilize on Seattle streets on May 1 each year? We have your May Day questions covered. What is May Day? Depending on where you live, May Day is either an ancient European spring festival or a day of support for immigrant and workers’ rights. In most nations, May 1 is an official public holiday known as International Workers’ Day or Workers’ Day. It's basically Labor Day for the rest of the world.