To many people, this means nothing. But of course 666 refers to the Beast in the Bible. Heaven knows what ten sixes means.
I don’t know who first noticed this looming numerical curiosity, but it was mentioned today on the Drudge report. To see the projection, go to the U.S. Census Bureau’s World POPClock Projection page. There you’ll find these projections among others:
So sometime in between May 1 and June 1, the gaggle-of-sixes milestone will be passed.