May 1st was chosen in the late 19th century to commemorate the contributions of workers and labor unions. The first day of May was chosen to remember the victims of the Haymarket Massacre.
From Wikipedia:
The Haymarket affair (also known as the Haymarket massacre or Haymarket riot) was the aftermath of a bombing that took place at a labor demonstration on Tuesday May 4, 1886, at Haymarket Square[2] in Chicago. It began as a peaceful rally in support of workers striking for an eight-hour day and in reaction to the killing of several workers the previous day by the police. An unknown person threw a dynamite bomb at police as they acted to disperse the public meeting. The bomb blast and ensuing gunfire resulted in the deaths of seven police officers and at least four civilians; scores of others were wounded.
