Americans of my generation grew up hearing this line without necessarily knowing where it came from. Turns out it’s from an epic retelling of Arthurian legend by the 19th-century American poet James Russell Lowell. This is the passage I used to hear the old-timers recite:
And what is so rare as a day in June?
Then, if ever, come perfect days;
Then Heaven tries earth if it be in tune,
And over it softly her warm ear lays;
Whether we look, or whether we listen,
We hear life murmur, or see it glisten …
Here’s a short article in the Paris Review with pretty much all you need to know about it, “one of those improbable bits of poetry that’s somehow managed to survive in our cultural memory.”