Analysis of An Unmarked Festival

There's a feast undated yet:
Both our true lives hold it fast,--
The first day we ever met.
What a great day came and passed!
--Unknown then, but known at last.

And we met: You knew not me,
Mistress of your joys and fears;
Held my hands that held the key
Of the treasure of your years,
Of the fountain of your tears.

For you knew not it was I,
And I knew not it was you.
We have learnt, as days went by.
But a flower struck root and grew
Underground, and no one knew.

Days of days!    Unmarked it rose,
In whose hours we were to meet;
And forgotten passed.    Who knows,
Was earth cold or sunny, Sweet,
At the coming of your feet?

One mere day, we thought; the measure
Of such days the year fulfils.
Now, how dearly would we treasure
Something from its fields, its rills,
And its memorable hills;

--But one leaf of oak or lime,
Or one blossom from its bowers
No one gathered at the time.
Oh, to keep that day of ours
By one relic of its flowers!