 -- President Donald Trump marked the unofficial end of summer by wishing the country a happy Labor Day on Twitter.

"We are building our future with American hands, American labor, American iron, aluminum and steel. Happy #LaborDay!" he wrote.

We are building our future with American hands, American labor, American iron, aluminum and steel. Happy #LaborDay! pic.twitter.com/lyvtNfQ5IO

Mentioning American workers and American-made products in his Labor Day tweet reflects the president's long-running focus on the nation's domestic industry and employment. He has regularly touted the U.S. unemployment rate, which currently sits at 4.4 percent, as one of his administration's successes since taking office in January. But Trump has also come under fire, particularly during his "Made in America Week" earlier this summer, for the fact that many of the goods from his family's businesses are produced abroad.

Labor Day was first celebrated as a national holiday in 1894 after a number of states established days to honor union workers. President Grover Cleveland signed a law designating the federal holiday less than a week after the end of the Pullman strike, in which railway workers in Chicago walked off the job to protest pay reductions.