In many parts of Europe, May Day is still celebrated as the onset of spring with picnics and outdoor barbecues. But across the world, May 1 is also traditionally marked with strikes, parades and occasionally violent protests.

Major demonstrations have been held in recent years in Havana, Cuba; Istanbul, Turkey; and Caracas, Venezuela, where police and opponents of President Nicolas Maduro traded tear gas, rocks, and petrol bombs last year. This year, workers in countries around the world marked International Workers’ Day, also known as Labor Day or Workers’ Day, with strikes and rallies for better working conditions...