Forum: ell.stackexchange.com
Tags: <prepositions><difference><time>
Title: Do "once a year" and "once in a year" mean the same thing?
Question: I've seen  people using the phrase  once a year  but I wonder isn't it should be  once in a year . Are they both the same and acceptable?

Answers:
sents: text: There is a slight, but meaningful, difference.
text: "
text: Once a [time period]
text: " implies frequency observed over a range of time greater than [time period].
text: "Once in a [time period]" implies an observed frequency of 1 in a single [time period].
text: If somebody were to say something occurred once in a month then he means he is talking about an event with a time of a month.
text: If someone says something occurs once a month then he definitely has data larger than a month.
text: Example: My last boyfriend, Eric, would buy me flowers once a month!
text: But John has only bought me flowers once in a year.
text: Does John hate me or is he just less sentimental and thus will only buy flowers for year anniversaries?
text: Another example: During the first year of our relationship, Phillip bought me flowers once a month.
text: Why would he now only buy me flowers once in a year?
text: As FumbleFingers brings up in the comments, the "once in a [time period]" could instead be an application of figurative speech and not based on technical data.
text: The following example shows a usage of the phrase that indicates exaggerated speech is at play.
text: Wow!
text: That's rare; it only happens once in a blue moon!
text: This is perhaps more indicative of a common idiom but still important to bring up.
text: To slightly elaborate on what was established above, it may not be obvious that the phrase "once in a [time period]" can be used to establish a cap on repetitions in a [time period].
text: See below.
text: Alice:
text: "So, he goes to Mexico 10 times a year?"
text: Bob: "
text: Yes, but never twice in the same month; he only goes once in a month."
answer_details: author: ell.stackexchange.com/users/41593/Harrichael
score: 38
sents: text: The main reason they're "not the same" is because of this huge difference in frequency of use ...
text: I don't think there are any many 1 contexts where you need to include in , so
text: the easiest option is never to use it (because in most contexts it will sound odd to native speakers if you include in ).
text: But as often happens in English, where one of two different versions of an expression becomes dominant, the less-favoured alternative may nevertheless survive, and start to carve out its own "semantic territory" and associations.
text: Take, for example... 1:
text: It doesn't snow here often.
text: Usually not more than once in a year.
text: 2:
text: She doesn't go out much.
text: Probably not once in a month.
text: There's nothing "unidiomatic" about those usages (though they'd be fine without the extra preposition).
text: I think the reason they're acceptable is because they stress the infrequency of the event (it's rare, uncommon ).
text: Another important factor is that the occurrence rate being referenced has to be somewhat imprecisely measured, unpredictable, irregular .
text: So (for me at least)...
text: 3:
text: I've always had regular checkups.
text: I see my dentist once in a year ...
text: doesn't work at all.
text: 1 Note that in has semantic significance in constructions like
text: I haven't seen him once in a year , where it means once in the past year (i.e. - the twelve months immediately preceding time of utterance).
text: There are also closely-related usages such as once in a lifetime, once in a blue moon where in is idiomatically required.
text: As with the examples above, all these contexts imply infrequent / sporadic .
answer_details: author: ell.stackexchange.com/users/126/FumbleFingers Reinstate Monica
score: 13
sents: text: It's important to remember: the word a can be used as a preposition, meaning per .
text: See this screen shot focusing on just a few of the many definitions found on Wordnik :
text: Therefore, the phrase once a year is perfectly grammatical, acceptable, and idiomatic.
answer_details: author: ell.stackexchange.com/users/113/J.R.
score: 9
sents: text: Once a year is considerably more common: here is an NGram .
text: It implies that something takes place once in every calendar year.
text: For example: I take a holiday in Norway once a year Once in a year is much less common and is in decline: many of the older references relate to annual religious observances.
text: In modern usage, it refers to something that happened only once within a one-year period which need not necessarily be a calendar year, for example:
text: He bought me a drink only once in a year.
answer_details: author: ell.stackexchange.com/users/30978/JavaLatte
score: 7
sents: text: Once a year/month/week would be more common.
text: Once in a year/month/week would be understood, but less likely used.
text: You are more likely to hear "Once in the past year" or "Once in the past five months" than "once in a year" For an expression of the form "once a ", "a" acts as a preposition meaning "per"; it could also be defined as "each"
answer_details: author: ell.stackexchange.com/users/12503/eques
score: 4
sents: text: I think that there's a precision difference between those two phrases.
text: "
text: Once a year" says to me that every year, about the same time, something happens.
text: He goes on vacation in Florida once a year.
text: - It's some sort of a tradition, which is usually tied to a particular time.
text: He only goes to Florida once in a month.
text: - It's more rare to hear this, especially longer than a month, and it's a "may or may not happen" sort of occurrence.
answer_details: author: ell.stackexchange.com/users/41596/BenPen
score: 3
sents: text: In addition to what has already been explained, it would be presumed that an event which happens once a year probably occurs around the same time.
text: Valentine's Day only comes once a year.
answer_details: author: ell.stackexchange.com/users/41632/EnglishTeacherEric
score: 1
sents: text: "Once a year" denotes something recurrent that happens each period. "Once in a year" denotes something happened in the last year.
text: "I used to party once a week, but since I married I went to a party once in a year."
answer_details: author: ell.stackexchange.com/users/34967/roetnig
score: 1
sents: text: From my perspective this is the difference in meaning: Once in a year: The expectation is that it happens each year and 1 time each year Once a year: The expectation is that it happens at a 1 year interval Example
text: This difference is usually not relevant, but here is an example where only 1 of the two seems appropriate: Someone whose birthday is around 1 january, may celebrate it in december or january.
text: In this case it would be strange (wrong?!)
text: for the person to say: Once in a year I celebrate my birthday
answer_details: author: ell.stackexchange.com/users/41673/Dennis
score: 1

Summary of the answers:
There is a slight, but meaningful, difference. Once a year" denotes something recurrent that happens each period.  "Once in a year" denotes that something happened in the last year.
There is a small difference between the two. "Once in a month" refers to an event  with a time of a month, but for "once a month" the speaker's data is larger than a month. Another difference is frequency of use, with "once a month" being much more commonly used. There is also a difference in precision. "Once a month/year" means that the event probably happens around the same time every month/year.