For a bit of history see The Roots of May Day, from Nelson Lichtenstein, posted at SLATE.COM the same say. Lichtenstein is a professor of history up the coast at UC Santa Barbara, where he directs the Center for Work, Labor, and Democracy.

Here's a bit of what he has to say -

... These May Day demonstrations and boycotts return the American protest tradition to its turn-of-the-20th-century ethnic proletarian origins - a time when, in the United States as well as in much of Europe, the quest for citizenship and equal rights was inherent in the fight for higher wages, stronger unions, and more political power for the working class.

Because today's marches are on a workday, they recall the mass strikes and marches that turned workers out of factories that convulsed America in the decades after the great railway strike of 1877, the first national work stoppage in the United States. Asserting their citizenship against the autocracy embodied by the big railroad corporations, the Irish and Germans of Baltimore and Pittsburgh burned roundhouses and fought off state militia in a revolt that frightened both the rail barons and the federal government. Hence the 19th-century construction of all those center-city National Guard armories, with rifle slits designed to target unruly crowds. The protesters wanted not only higher pay and a recognized trade union but a new birth of egalitarian freedom. Indeed, May Day itself, as an international workers holiday, arose out of a May 1, 1886, Chicago strike for the eight-hour workday. The fight for leisure - clearly lost today - was a great unifying aspiration of the immigrant workers movement a century ago with its slogan, "eight hours for work, eight hours for sleep, eight hours for what we will."

The largest mobilization of immigrant workers in U.S. history occurred in 1919, when President Woodrow Wilson's rhetorical celebration of self-determination and "industrial democracy," or self-rule at the workplace, echoed across steel districts from Homestead, Pa., to Gary, Ind. Strike organizers printed their handbills in 15 different languages. Immigrant churches and working-class lodge halls served as soup kitchens. The strikers called the mounted police "Cossacks." All these eruptions, which would successfully Americanize millions of immigrants in the 1930s, blended trade unionism, ethnic self-consciousness, and the demand for full citizenship. That unity proved essential for a long season of New Deal hegemony. And that's why this spring's awakening of a new generation of immigrant working-class half-citizens holds such promise for liberals.

The last of these great labor-strike demonstrations came in 1947. On an April workday, the United Automobile Workers flooded Detroit's Cadillac Square with more than a quarter million of its members to protest congressional enactment of the Taft-Hartley Act, which curbed union strike power and disqualified radicals from labor leadership. Most laborites called Taft-Hartley a "slave labor law." Then as now, the leaders of the demonstration were divided over tactics. The left, and not just those oriented toward the Communists, wanted to shut down the factories so that American unions could deploy, as one top UAW officer put it, "the kind of political power which is most effective in Europe." More cautious unionists, led by UAW President Walter Reuther, sought a huge demonstration but one that began only after workers clocked out for the day. Capitalizing on these internal divisions, and on the early Cold War hostility to labor radicalism and political insurgency, the auto companies took their pound of flesh. They fired key militants and cut off the tradition of white, working-class strike demonstrations in industrial cities for the rest of the 20th century.

For our generation, as for the one before it, the idea that we might change the conditions of work life and the structure of politics has seemed either radical fantasy or Parisian self-indulgence. Celebrations of May Day, the holiday that embodies that imagined link, have been consigned to the most self-conscious and marginal radicals. In 1958, President Dwight D. Eisenhower proclaimed May 1 "Law Day" so as to snuff out any proletarian embers that might have continued to smolder through the Cold War.

The 1960s civil rights and anti-war movements kept their distance from workplace actions, which became the province of an increasingly stolid and constrained trade unionism. The protests of that era were almost always held on weekends. The 1963 March on Washington, where Martin Luther King Jr. gave his "I Have a Dream" speech, took place on a sweltering Saturday afternoon. There were plenty of protest signs paid for by the union movement, but no factories shut down that day. The same is true of the big anti-war marches, and American feminists and gay-rights advocates have continued that tradition. The linkage between workplace protest and civil engagement has been broken - one reason that the boycotts and work stoppages today seem so novel and controversial.

When weekday work stoppages did take place, their marginality, and even alienation, from mainstream America was revealing. Arab workers put down their tools in June 1967 to protest U.S. support of Israel in the midst of the Six Day War. Millions of black workers left work when they learned of MLK's assassination on April 4, 1968, but black power efforts to use the strike to build a radical movement on the assembly lines largely failed in Detroit a year later. Today's marches and boycotts are restoring to May Day something of its old civic meaning and working-class glory. Even some of the most viciously anti-union employers of Latino labor, like Perdue, Cargill, and Tyson Foods, kept their factories closed. As in the crucial struggles that began more than a century ago, today's marches have forged a link among working-class aspiration, celebrations of ethnic identity, and insistence on full American citizenship. It's an explosive combination. And it could revive and reshape liberal politics in our time.

Perhaps so. So consider this image.