A law requiring large companies to give workers 60 days notice of plant closings or mass layoffs takes effect Saturday, with labor calling it "a day of fairness" and business denouncing it as unprecedented meddling.

"I think it's one of the major victories for working people in the last eight years, and maybe longer," said Sen. Howard Metzenbaum, D-Ohio, the bill's chief sponsor.The law requires employers with at least 100 workers to give the employees - and their communities - at least 60 days notice of plant closings and large layoffs that affect 50 or more workers at a job site.

The new law was the subject of a bitter battle between business and labor interests and then between Congress and President Reagan last year. It also requires that two months notice be given if a layoff of more than six months duration results in 50 or more workers losing their jobs - if the 50 people make up at least a third of the workers at a site. Sixty days advance notice also must be given if at least 500 jobs are lost due to a layoff of more than six months, whether or not the 500 people constitute one-third of the work force at a site.