Douglas Harper's Etymology Dictionary
1667, Latin, literally "wonderful year, year of wonders," title of a publication by Dryden, with reference to 1666, which was a year of calamities in London (plague, fire, war).
Wiktionary
n. Any given year that undergoes a multitude of significant and fateful events, be they disasters or successes.
Wikipedia
Annus mirabilis is a Latin phrase that means "wonderful year", "miraculous year" or "amazing year". This term was originally used to refer to the year 1666 (see below), and today is used to refer to several years during which events of major importance are remembered. Prior to this, however, Thomas Dekker used the phrase mirabilis annus in his 1603 pamphlet The Wonderful Year, "Wherein is shewed the picture of London lying sick of the plague."
Annus Mirabilis is a poem written by John Dryden published in 1667. It commemorated 1665–1666, the "year of miracles" of London. Despite the poem's name, the year had been one of great tragedy, including the Great Fire of London. Johnson writes that Dryden uses the term "year of miracles" for this period of time to suggest that events could have been worse. Dryden wrote the poem while at Charlton in Wiltshire, where he went to escape one of the great events of the year: the Great Plague of London.
Usage examples of "annus mirabilis".
His annus mirabilis, the plague years-he discovered all the basics of modern science, the laws of motion, optics, calculus, gravity.
This was more than a little ironic since it was he, in his annus mirabilis of 1905, who had so persuasively explained how photons of light could sometimes behave like particles and sometimes like waves—.