An anniversary is usually a word used to commemorate things that happen over 12 months .However,in English we tend to use it to commemorate things or events that have been true or in existences for less than 12 months .In your instance you could say :
-It's our six-month anniversary
or
-We have been together for half a year
"anniversary" は、通常1年以上のことを[祝う](https://eikaiwa.dmm.com/uknow/questions/34519/)ときに使われます。ただ英語では、1年以下のことを祝う場合にも "anniversary" を使う傾向があります。このケースでは以下のように言えます：
-It's our six-month anniversary
私達の6ヶ月の[記念日](https://eikaiwa.dmm.com/uknow/questions/32625/)です
-We have been together for half a year
[付き合って](https://eikaiwa.dmm.com/uknow/questions/33664/)半年になります
Well, you can't call it an anniversary because anniversary (from the Latin 'anno') refers to a yearly event. So you just have to form your own description of what this event is.
I would say six month anniversary because half a year is six months. It is six months after you began dating and therefore you are celebrating those six months. Half-year is acceptable as well, but six month is more common. Anniversary typically refers to whole years, however, people commonly say one-month anniversary or in your case six month. Happy six-month anniversary!
"Happy 6 months anniversay"
This is probably the most common and maybe the only way to express this feeling in words ofcourse. Since anniversaries are celebrate on an annual basis hence the word anniversary there is no real other way of saying this. Knowing all this most people dont care and still use the word anniversary after the number of months they are celebrating i.e. 3,6,9 etc.
Normally, anniversaries are celebrated on a yearly basis. However, if there is a time that is being celebrated before, then you can just say the amount of time and then anniversary. For example, 10 months anniversary.
"Happy six months anniversary!"
"lets celebrate our Half a year anniversary!"
six months is a more common term used opposed to referencing 'half a year'.
"Happy six months anniversary!"（半年記念日おめでとう）
"Let's celebrate our Half a year anniversary!"（付き合って半年を祝おう）
'six months'（半年）の方が 'half a year'（半年）よりも一般的です。
"half year mark" anniversary has part of the latin word, Annual which means every year, on a yearly basis. the half year mark indicates half of a year, or 6 out of the 12 months.
"6 months left to go" means you're at the half year point of a year mark, you're 6 out of 12 months into the year and you have 6 months more until a year.
"We're half way through our 1 year" means 6 month anniversary, 6 months until 1 year.
「half year mark」-「anniversary」にはラテン語が含まれています。「Annual」は毎年という意味で、「half year mark」は半年（6ヶ月）という意味です。
「6 months left to go」は、半年の地点にいる、12ヶ月のうち6ヶ月が過ぎ、1年まで残り6ヶ月あることを表します。
「We're half way through our 1 year.」は、6ヵ月の記念日、一年まで6ヵ月という意味です。
Although an anniversary typically celebrates something lasting for an entire year, we can also celebrate different milestones that are less than a year. In this example if we are talking about a relationship that has lasted half of a year we would call this a, "six month anniversary."
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