They are the days, some tragic and some triumphant, that make the "United" in "United States" more than a political slogan.
They are exceedingly rare, these days.
A compelling case can be made that America has experienced just three such days in the past half century.
One of those happened exactly 50 years ago today - Nov. 22, 1963, when President John F. Kennedy was assassinated in Dallas.
On July 20, 1969, the nation again watched, equally transfixed but this time by joy and awe rather than sadness, as Neil Armstrong stepped onto the surface of the moon.
On Sept. 11, 2001, we looked on, incredulous, as the Twin Towers crumpled.
What these days have in common, besides engraving their scenes in our collective memory, is that each almost instantly suppressed the grievances and societal debates that characterize our free society.
We shared, however briefly, an experience that elevated our commonality as Americans above our party affiliations or political beliefs.
We get back to bickering, of course.
This annoys us at times, to be sure.
But would we really prefer a different system?
We rail against the seemingly juvenile obstinance in Congress, and wonder why we ever elected these bozos.
But ultimately we appreciate that we can replace the bozos if they finally exceed our patience, that we can influence the direction of our country.
It's unfortunate that a monumental tragedy sometimes is needed to remind us of these truths.
But on days such as today, when we remember one of these terrible events - one now two generations in our past - perhaps we can muster just a bit of that national unity.
We can resume our important arguments about Obamacare and other matters tomorrow. But for today we ought to celebrate America, and Americans.