Somewhat apropos today, the solstices/equinoxes are by their nature celebrated at the same time of year universally. Granted, not everyone celebrates them, but you couldn't ever get everybody to celebrate anything.
Though if we're getting pedantic (like the head commenter of this thread), the solstices are opposites in the northern vs southern hemisphere, and on the equator solstices and equinoxes are irrelevant.
I do think New Year's on the Gregorian Calendar is up there, and the soccer World Cup does well, too.
I think there is some sort of celebration of Gregorian New Year's in most of the world, even if it's not as big as lunar new year and other local calendar systems.