The poet is George Buchanan (1506–1582), the poem “Calendae Maiae.” De Selincourt
gives the Latin lines and supplies this translation from Peter Hume Brown’s 1890 Buchanan

When, still rejoicing in her birth,
Spring brightened all the new-made earth
And in that happy golden age
Men knew no lawless passions rage,
Thy train of joys embraced the year:
Soft breezes wooed the untilled field
Its blessings all unforced to yield.
Even in such mildest atmosphere
For ever bask those happy isles,
Those blessèd Plains, that never know
Life’s slow decay, or poisoned flow.
Thus ‘mid the still abodes of death
Should steal the soft air’s softest breath,
And gently stir the solemn wood
That glooms o’er Lethe’s dreamless flood.
And, haply when made pure of stain
By cleansing fire, the earth renewed
Shall know her ancient joys again,
Even such mild air shall o’er her brood.