une fois par année/an - what's the difference?

Question

I think I understand the general difference between année and an, but here's one example which got me confused because both seem just as likely.

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"C'est un événement à ne pas manquer, car ça n'arrive qu'une fois par année."

"C'est un événement à ne pas manquer, car ça n'arrive qu'une fois par an."

Answer 1

Questions about an vs année are recurrent in FSE. See for example Can “an” and “année” be used interchangeably, is there a difference in meaning?

In your specific case, the most common usage is the second one (une fois par an) but the first one shouldn't be ruled out.

If the year it is about is not a calendar year but some period of one year, like une année scolaire or une année fiscale, the first one would be equally possible.

If an adjective like scolaire, fiscale, civile, calendaire, entière, complète... is used, then année is mandatory; e.g.:

"C'est un événement à ne pas manquer, car il n'arrive qu'une fois par année scolaire.

Answer 2

In your example, qu'une fois par an is expected, as an is used as a unit of time. It's also possible to use année, denoting an individual period of time, but you'd have to phrase it qu'une fois chaque année (once each year).

Answer 3

Both are correct French; this can be asserted since both expressions are found before a full stop, as can be verified in the ngram. However, "une fois par an" is very common in written material, whereas the usage of "une fois par année", which is far from rare but yet far from being as important, could be due to the greater freedom in the choice of words that is characteristic of writing, and therefore this means that the frequency in the spoken language could be lower still. Nevertheless, both can be used in speaking. Personally, I wouldn't find "année" odd. I would find "une fois l'année" very unusual, even though it is still correct and I wouldn't use this form, keeping to "une fois l'an". Notice from this ngram that "une fois par an" is much more common than "une fois l'an".