It's the first Monday in September, when the United States observes Labor Day by avoiding labor. Today is a holiday north of the border too, but in Canada it's called Labour Day. Labour, of course, is the accepted spelling in the United Kingdom and Commonwealth countries like Canada. Americans prefer labor to labour, just as they prefer color, favor, honor, humor, neighbor, and a few dozen other words ending in -o(u)r. How did the spellings diverge?

As with so many distinctly American spellings, Noah Webster gets a lot of the credit — though not all of it. Many of the spelling reforms pioneered by Webster had their roots in earlier orthographic innovations on both sides of the Atlantic. In the case of -our words changing to -or, the groundwork was laid in the seventeenth century. Most words ending in -our had entered English from French after the Norman Conquest and stayed that way in words of two syllables. In longer words, the -u- tended to be dropped, and some felt that the two-syllable words should be regularized too. In one of the early English dictionaries, Thomas Blount's Glossographia of 1656, the shift to -or was already evident in a few words: Blount had error instead of the earlier errour, armor instead of armour, and ill-favored instead of ill-favoured.

Dictionaries weren't very settled in their spelling, however, before Samuel Johnson endeavored (or endeavoured) to provide a more consistent rendering of English in his landmark Dictionary of the English Language (1755). As David Wolman, author of Righting the Mother Tongue, explained in our interview with him, Johnson was rather conservative in spelling matters, at least early in his lexicographical career. So Johnson insisted on spelling fantastic as fantastick and magic as magick, even though the -ick variants were already falling out of style. Similarly, he chose to go with errour instead of error, since he saw the latter as an unnecessary kind of phonetic spelling.

Johnson belittled these new-fangled spellings, but in doing so helped shed light on the proposed orthographic reforms of his day:

We have since had no general reformers; but some ingenious men have endeavoured to deserve well of their country, by writing honor and labor for honour and labour, red for read in the preter-tense, sais for says, repete for repeat, explane for explain, or declame for declaim. Of these it may be said, that as they have done no good, they have done little harm; both because they have innovated little, and because few have followed them.

But the early spelling reformers so disdained by Johnson did have some followers in later years, Noah Webster being chief among them. Like Johnson, Webster sought to stabilize the inconsistencies of English spelling, but Webster was much more willing to introduce "rational" spelling reforms based on the phonetic shape of words. In the preface to his 1806 Compendious Dictionary of the English Language, he justified the -or spelling and made his break from Dr. Johnson:

To purify our orthography from corruptions and restore to words their genuine spelling, we ought to reject u from honor, favor, candor, error, and others of this class. Under the Norman princes, when every effort of royal authority was exerted to crush the Saxons and obliterate their language, the Norman French was the only language of the English courts and legal proceedings, and the Latin words which, at that period, were introduced into use in England, came clothed with the French livery... Hence for some centuries, our language was disfigured with a class of mongrels, splendour, inferiour, superiour, authour, and the like, which are neither Latin nor French, nor calculated to exhibit the English pronunciation. Johnson, in reverence to usage, retained this vitious orthography, without regarding the palpable absurdity of inserting u in primitive words, when it must be omitted in the derivatives, superiority, inferiority and the like; for no person ever wrote superiourity, inferiourity. A sense of propriety however, has nearly triumphed over these errors; and our best writers have almost unanimously rejected the u from this whole class of words, except perhaps ten or twelve.

Thus the idea of writing labour as labor, along with similar spellings, wasn't original to Webster, but he was a powerful advocate for their adoption. His Compendious Dictionary had significant repercussions on the direction of American orthography. Even Webster's bitterest rival in the dictionary trade, Joseph Emerson Worcester, accepted the change of -our to -or, though he rejected most of Webster's other Americanizations. Webster's impact wasn't immediate, but by the mid-nineteenth century American spelling had firmly and irrevocably switched over to the -or style. To this day, it remains one of the starkest differences between American and British English.

Here you can find a list of words that Americans spell with -or and Brits spell with -our. For more on the topic of -o(u)r, see Chris M. Anson's 1990 article in the International Journal of Lexicography, "Errours and Endeavors: A Case Study in American Orthography." And for the influence of Webster on American spelling in general, see David Micklethwait's Noah Webster and the American Dictionary (2005).