Labor Day was once the nation’s most blatantly political national holiday — created by the trade-union movement to celebrate the right of working people to bargain collectively and to stage strikes to press their demands.

The Post was an early and vigorous supporter of the movement. As early as 1836, editor William Cullen Bryant wrote: “Strike the right of associating for the sale of labor from the privileges of a freeman, and you may as well at once bind him to a master.”

Even before Congress created the federal holiday in 1894, New York hosted the nation’s first Labor Day parade when 10,000 workers took off from their jobs to march from City Hall to Union Square. As the movement grew, so did the parades and celebrations.

But times have changed. Today, Labor Day is largely an occasion time for sales, end-of-summer cookouts and back-to-school preparations. Why? Because the movement has sharply diminished and dramatically changed.

In 1954, more than one in three American workers was a union member. Today, it’s less than one in 16 private-sector workers — but, in a huge shift, more than one-third of public-sector workers. Indeed, 48 percent of all union members now work for government.

Yet even as pro-union a president as Franklin Delano Roosevelt — who did more than any other chief executive to extend organized labor’s reach — was certain that unions had no place in government service. As he wrote in 1937: “All government workers should realize that the process of collective bargaining, as usually understood, cannot be transplanted into the public service.”

And strikes by public employees, he wrote, are “unthinkable and intolerable.”

The way FDR’s warning has gone by the wayside is probably a major reason why public support for unions is way down — and why huge Labor Day parades are a distant memory.

But a day of thanks and a public salute still is due to all those working men and women who — in the words of one of the holiday’s originators, AFL co-founder Peter McGuire — “from rude nature have delved and carved all the grandeur we behold.”