The holiday is most often associated with the commemoration of the social and economic achievements of the labor movement. The May 1st date is used because in 1884 the Federation of Organized Trades and Labor Unions demanded an eight-hour workday in the United States, to come in effect as of May 1, 1886. This resulted in the general strike and the U.S. Haymarket Riot of 1886, but eventually also in the official sanction of the eight-hour workday.
http://en.wikipedia.org/wiki/May_Day