During the 20th century, May Day was celebrated throughout Europe as the holiday honoring the industrial worker.
This May 1 has a different significance. It is the day on which the European Union will expand from 15 to 25 members with the admission of Cyprus, Malta and eight former Communist countries of Central and Eastern Europe: the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia.
Yet Saturday may turn out to mark not the beginning of a new era for Europe but the end of the era that began in 1950, when France and Germany agreed to pool their coal and steel industries. That period was an era of integration in which the nations of Western Europe drew ever closer both economically and politically.
That five-decade process is now likely to stop, frustrating those who hope for the creation of a single European superstate. But the end of integration may pave the way for Turkey and Russia ultimately to join the EU, which - by fostering political stability and economic growth in those two countries - would be good for Europe, good for the United States and good for the world.
There are several reasons that the expansion of the EU from 15 to 25 members will make further integration unlikely. One is sheer numbers. The original European Coal and Steel Community, with France and Germany at its core, included only six member countries, which could work easily together. An organization of 25 will be far more unwieldy and difficult to manage.
Moreover, the new members differ from the older ones in a number of ways, the most important being that the Central and Eastern Europeans are considerably poorer than their Western European partners. This means that most are not now, and will not soon be, eligible to join the EU's most important cooperative project, its shared currency, the euro.
Their relative poverty means that the most pressing task for the new members is to achieve rapid economic growth. They have performed in lackluster fashion over the last five years, with Poland, Slovakia, Hungary and the Czech Republic averaging a disappointing 3 percent per year. But the EU itself has done even worse. This year it is expected to grow by only 2 percent, less than half the 4.5 percent anticipated for the United States and the world economy as a whole.
The reason for the EU's sluggishness is the thick network of regulations and subsidies that have accumulated over the past half century. The new members are likely to conclude that these are holding them back economically, and try to abolish some of them. This will prove divisive, stalling the process of integration.
Already divisive is the new constitution that has been drafted to govern the expanded EU. Eight member countries have indicated that they will take the extraordinary step of holding a national referendum on the document, rather than simply submitting it to their parliaments for ratification.
Among them is Britain, where skepticism about closer political and economic integration runs high and where public repudiation of the new constitution would deal a crushing blow to efforts to move the EU in the direction of a single European superstate.
While dismaying the advocates of such an outcome, however, the end of European integration could open the way for a more important and useful development - the inclusion in the EU of two major countries that, for the present, have been excluded from it: Turkey and Russia.
Both countries are widely regarded within the EU as too large - each has more people and territory than Germany, currently the EU's biggest member - and too different economically and culturally from the countries of Western Europe to fit into a tightly knit, politically coherent European Union.
But the more loosely organized EU that the May Day expansion seems likely to create might be able to accommodate the two, to the benefit not only of Turkey and Russia themselves but of all countries, including the United States, that have an interest in Turkish and Russian political stability and economic prosperity.
Michael Mandelbaum is a senior fellow at the Council on Foreign Relations and a professor of foreign policy at the Johns Hopkins School of Advanced International Studies.