(CNN) To most people in the Northern Hemisphere, May Day conjures images of brightly colored twirling ribbons and promises of warm days ahead. That's not the whole story, though: May Day is also a day of protests and riots that traces its modern roots back to a world-changing explosion in Chicago.

When is May Day?

May Day is May 1 every year. Easy to remember, right?

What is May Day?

Depending on where you are, it's either a seasonal celebration or a day to celebrate workers' rights, or maybe a little bit of both. Think of the latter use as a Labor Day, if you will, for the rest of the world.