How can there be a six-month anniversary of anything? A six-month mark, sure, but an anniversary always means the same calendar date, doesn’t it?
when it comes to tragedies, I think that they use the date when the memorial builders contract is first given.
Misnomer it is, but lately, people have come to call “milestone dates” anniversaries regardless of whether or not they actually mark a year’s passage.
the semianniversary?
Repeat after me:
“The root of a word is not its meaning.”
It doesn’t matter that the root of “anniversary” once meant “year.” It has absolutely nothing to do with the meaning of the word.
“The root of a word is not its meaning.”
You get paid a salary. Do you object to the fact that they are paying you in money and not salt?
“The root of a word is not its meaning.”
Sailor orient themselves by the North Star. But the North Star isn’t in the East.
“The root of a word is not its meaning.”
Would you adjective a baseball? But the original meaning of “adjective” is “to throw.”
“The root of a word is not its meaning.”
Get the idea?
People talk of “one-month anniversary” or “six-month anniversary” all the time. And listeners know why they mean. Why?
Say it: “The root of a word is not its meaning.”
OK. How about this: “The etymology of a word is not necessarily relevant to the proper usage of a word.” No argument, but irrelevant.
The fact that one might understand the intended meaning does not change the incorrect usage of a word. Merriam-Webster defines anniversary as “the annual recurrence of a date marking a notable event.” (Emphasis mine.)
“Six-month anniversary” is therfore incorrect usage. It is twisting a perfectly good word with a distinct meaning due to mental laziness. What’s wrong with “commemoration”?
But, **RealityChuck ** I adjective baseball all the time. Usually when my team is losing. 
But if you’re really longing for a word, someone suggested “lunaversary” here a couple years ago, and I liked it and have used it ever since
Yes, Maralinn, according to the dictionary definition of ‘anniversary’, the phrase ‘6 month anniversary’ seems flawed.
However, as a way of communicating an idea, i.e. that it’s 6 months since the events of Sep 11th, most people would find it readily understandable.
If you accept the view that language is simply a tool for communicating, then all that matters is whether the commmunication succeeds. If A is trying to communicate with B and does so successfully, the language has done its job. If not, it hasn’t.
By this criterion, the phrase ‘6 month anniversary’ seems to work for most people.