Chris Ambidge wrote to report a mailing from the San Francisco LGBT Community Center, celebrating its ten-year anniversary; on-line story here. Chris would have said tenth anniversary, and found ten-year anniversary redundant (like PIN number, he said), because ‘year’ is already contained in anniversary. This is one of two complaints about the usage of anniversary: the perceived pleonasm of n-year anniversary. The other complaint is about the perceived contradiction in n-span anniversary for spans other than year, especially month (one-month anniversary, six-month anniversary, six-week anniversary) — again, because ‘year’ is contained in anniversary. The second complaint seems to be the older one; it’s the only one reported in MWDEU (in 1989).
The second innovation presumably arose from a weakening appreciation of the etymology of anniversary, so that the word can be extended to recurring spans of time other than a year (though the default span of time was still a year). Then people began supplying year, for clarity, giving us things like ten-year anniversary, as distinct from ten-month anniversary and ten-week anniversary.
And the complaints piled up.
MWDEU‘s earliest evidence for the n-span variant came in correspondence who wrote in 1967 to say that it was “not uncommon in speech”, but at the time they had only one example in print, from 1941, and they concluded that the usage was primarily spoken. Their first recorded complaint is from James J. Kilpatrick in 1984. Two on-line complaints hammering home the etymological point:
 Common Errors in Written English: Monthly Anniversary
To use the term anniversary in conjunction with the word month is contradictory. Anniversary means the return of an annual event, not a monthly event.
Sadly, the pressure of popular (but mistaken) usage seems to bring allowance for this contradictory expression. At least one trusted dictionary [the Free Merriam-Webste] refers to the broad usage of anniversary:
“the annual recurrence of a date marking a notable event; broadly : a date that follows such an event by a specified period of time measured in units other than years <the 6-month anniversary of the accident>” (link)
 This is another one of those words so universally misused that it may be time to throw in the towel. Problem is, that’s not my nature.
The respected Wall Street Journal reported something about a “six-month anniversary in Poland.” Impossible. All anniversaries have 12-month increments. The word means, literally, the yearly recurrence of the date of a past event. Its etymology (rooted in Latin) reveals this:
“Annus” means year, “versus” means “turn.” Yearly turn. (link)
Both sources insist that language should not change, that semantic extension should not be condoned, and that Etymology Rules. They both also recognize that common practice has moved past etymology and that they’re fighting a holding action. In cartoon form (although the cartoon drifts on to other matters after the first three panels), from Michael Zole’s Death to the Extremist:
[On the strip, from Wikipedia:
Death to the Extremist is a series of minimalist webcomics created by Michael Zole. Initially published in Hampshire College's magazine The Omen, it has been published regularly online since 2001. Currently, Death to the Extremist is updated every Tuesday and Friday.
The strip follows the strange, sometimes pie-oriented adventures of two amorphous entities, named One and Two, each of whom is represented only by a quarter circle.
One is a black quarter circle, labeled with the numeral '1'. He usually appears at the bottom-left corner of the panels in which he appears. Of One and Two, One is reportedly the 'fan favourite'.
Two is a white quarter circle with a black outline, bearing the label '2'. He usually appears in the bottom-right corner of each panel. Two does not have any arms. He is afraid of ponies.]
Broad anniversary is not in Brians’s Common Errors — but it’s also not (yet) in the OED.
In any case, there are thousands of ghits for one-month anniversary or one month anniversary: of couples, of a teen’s disappearance, of Trayvon Martin’s death, of a father’s death, of the Fukushima disaster, of an Occupy protest, etc. And quite a few for six-months anniversaries. You can get greeting cards.
Of the broad use of anniversary, MWDEU concludes:
The extension to a period of time other than a year does not seem especially irrational, however, in the absence of an alternative word for the idea.
That is, broad anniversary is useful. And, unless people start using it unmodified to refer to spans other than a year (in which case it would be ambiguous out of context), it’s unproblematic: so long as you’re willing to recognize that other people are using anniversary in ways different from your way, the meaning of things like one-month anniversary is clear.
Some people have wrestled to create alternatives to broad anniversary, via portmanteaus with anniversary — reduced to -versary in lunaversary or mensiversary or monthiversary ‘one-month anniversary’, or to -sary, as in these creations:
Now, for the development of “redundant” n-year anniversary. Here, the story has been well told by Ben Zimmer, in the NYT in 2010:
Is ‘One-Year Anniversary’ Redundant?
Neale Gifford writes: “One practice that annoys me is the use of ‘one-year anniversary’ or ‘five-year anniversary’ instead of ‘first’ or ‘fifth.’ Reason? Anniversary is derived from the Latin annus meaning ‘year.’ ” Ed Morman writes of “n-year anniversary,” “It’s not very mellifluous and it is, of course redundant. Help me bring back ‘nth anniversary.’ “
Gifford and Morman aren’t alone in their irritation. A few years ago, when The San Francisco Chronicle ran a headline reading, “Four-year anniversary draws protests,” an irate reader left a profanity-riddled voicemail that unkindly insinuated what substance could be found between the ears of Phil Bronstein, then the editor of The Chronicle. (The same reader was even more offended by the appearance of “pilotless drone” in the newspaper, and his recorded rant became something of a Web sensation.)
The New York Times, too, is hardly immune to this redundancy. The June 28 edition, for instance, featured an article on Dylan Ratigan‘s “one-year anniversary at MSNBC” and another stating that New York’s first gay pride parade commemorated “the one-year anniversary of the Stonewall uprising.” Philip Corbett, the newsroom’s style guru, has frequently complained about this usage in his “After Deadline” blog but laments that his reminders have “little effect.”
What has happened to the word anniversary? Even though the idea of yearly recurrence is built into the word etymologically, that idea has been clouded over centuries of use. And when an element of a word’s meaning becomes more opaque, redundancy is one method, however inelegant, that gets used to unmuddy the waters.
As the annual aspect has moved to the background of anniversary, the shift has opened the door for use of the term to mark the passing of shorter units of time. The 11th edition of Merriam-Webster’s Collegiate Dictionary, in its entry for the word, states that anniversary can refer more broadly to a date following a notable event “by a specified period of time measured in units other than years,” giving the example, “the 6-month anniversary of the accident.”
Linguists call this process “semantic bleaching”: the lessening of a word’s force through generalization. The bleaching of anniversary has been going on for quite a while, even if dictionaries are only now catching up. For more than a century, English speakers have been modifying anniversary with numbers of days, weeks or months. An article in the May 5, 1901, Atlanta Constitution described the Vanderbilt family’s Biltmore mansion in North Carolina, where little Cornelia Stuyvesant Vanderbilt “celebrated the three-months anniversary of her birth by planting out a tree on the estate.” A few years later, The San Jose Evening News reported on the unseemly nature of a widow marrying on “the third-month anniversary of her husband’s death.”
Anniversary has been pressed into service for nonannual commemorations in part because English has no other commonly used terms that can fill the gap. At various times since the 19th century, the monthly equivalent of anniversary has been dubbed a mensiversary, using the Latin root mensis for “month,” but this ad-hoc coinage has never caught on. (Members of the Facebook group “Make ‘Mensiversary’ a Word” continue to fight the good fight.) A more recent suggestion is the clunky English-Latin hybrid monthiversary.
With pressure on anniversary to expand its reach to subannual units, it’s no wonder that “nth anniversary” can somehow feel insufficient for traditional yearly celebrations, in need of the more explicit form, “n-year anniversary.” But if you don’t want to see the meaning of the word weakened any further, stick to the “nth” version and trust that others will have the good sense to discern anniversary‘s annualness.
Two final notes, on anniversary ‘wedding anniversary’ and birthday ‘birth anniversary’. Both expressions denote a date — a short-form date, just the month and day — and so can be modified by an ordinal: our tenth (wedding) anniversary, my seventieth birthday. OED2 on birthday gives two senses that are salient here:
1. The day on which any one is born …
2. The anniversary or annual observance of the day of birth of any one …
There are actually two subsenses for 1: a long-form date, including the year (September 6th, 1940), or a short-form date (September 6th). Birth anniversary as an alternative to birthday in sense 2 is well attested.