"Happy Labor Day"? According to the U.S. Department of Labor, it should be a happy day:
It constitutes a yearly national tribute to the contributions workers have made to the strength, prosperity, and well-being of our country.
Seriously, though, given the state of American labor unions in a post-industrial society, that sounds pretty silly. No one really thinks of Labor Day as having anything to do with the labor movement anymore. It's just a convenient day off at the end of the summer, the day the beaches close and the schools open (more or less).
In a nod to Labor Day sales, should the greeting be "Happy Bargain Hunting"?
What about the all-inclusive "Season's Greetings"? That one could work year-round.
Maybe we should adopt the Yom Kippur approach: "Have a meaningful Labor Day."
Or we could take after the British and rename it the Late Summer National Holiday.
The Brits already have the Early May Bank Holiday, Spring Bank Holiday and Summer Bank Holiday. When Christmas falls on a weekend, they can even celebrate Substitute Bank Holiday in lieu of 25th and Substitute Bank Holiday in lieu of 26th.
Try finding a greeting for those!