The
OSH Act, signed on December 29, 1970, went into effect on April 28, 1971. That explains this:
Presidential Proclamation -- Workers Memorial DayThe White House
Office of the Press Secretary
For Immediate Release April 28, 2010
Presidential Proclamation -- Workers Memorial Day
A PROCLAMATION
This year marks the 40th anniversary of both the Occupational Safety and Health Act and the Federal Coal Mine Health and Safety Act, which promise American workers the right to a safe workplace and require employers to provide safe conditions. Yet, today, we remain too far from fulfilling that promise. On Workers Memorial Day, we remember all those who have died, been injured, or become sick on the job, and we renew our commitment to ensure the safety of American workers.
....
NOW, THEREFORE, I, BARACK OBAMA, President of the United States of America, by virtue of the authority vested in me by the Constitution and the laws of the United States, do hereby proclaim April 28, 2010, as Workers Memorial Day. I call upon all Americans to participate in ceremonies and activities in memory of those who have been killed due to unsafe working conditions.