May Day, celebrated by workers across the globe as International Labor Day, falls on May 1.

But you'd be forgiven if that's news to you. While the day traces its origins to an American laborers' fight for a shorter work day, the U.S. does not officially recognize International Labor Day.

Like other countries that mark Labor Days on different dates, the U.S. and Canada celebrate their Labor Day in September.

U.S. resistance to celebrate International Labor Day — also called International Workers' Day — in May stems from a resistance to emboldening worldwide working-class unity, historians say.

"The ruling class did not want to have a very active labor force connected internationally," said Peter Linebaugh, author of The Incomplete, True, Authentic, and Wonderful History of May Day. "The principle of national patriotism was used against the principle of working-class unity or trade union unity."

That hasn't stopped American workers from commemorating the day, which in recent years has ranged from marching for labor rights to reading literature about Marxism.

"The meaning of that day keeps changing," Linebaugh said.