# once three days = every three days?

#### Kenny Chang

##### Senior Member
Hello, everyone.

Do the following sentences mean the same?
1. I go to the library once three days.
2. I go to the library every three days.

Note:
I was about to use "once two days/every two days" to make examples, but I found out that you wouldn't use "once two days." Why?

Thank you.

• 1. I go to the library once three days.
2. I go to the library every three days.
3. I go to the library once every three days.

We could say 'once in three days'. This means that sometime during the three days we go to the library. We might go on day 1 or day 2 or day 3.
We need 'in' to connect 'once' and 'three days'. We can't leave it out. Those little prepositions do a lot of work. They tell us what three days has to do with it. For instance, if you said 'once for three days' that would mean that three days was how long you were in the library..

'Once in three days' isn't the same as "I go to the library every three days." 'Once in three days' means that you always go on day 3. ===> Error corrected in post #7

Cross-posted.

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"I go to the library once every three days."

And "once" doesn't really mean it's done one time only, does it? It's just another way of saying "every three days", right?

What if she went to the library twice on the same day, say, before and after dinner and it happened every three days as well.
I guess I could still say "I go to the library once every three days.", couldn't I?

I wouldn't. If I went to the library on more than one occasion, I wouldn't say 'once.'

Whatever number you use in that sentence, it should equal the number of times you went to the library.

So these two do differ, don't they? We don't know the number of visits in the first one, right?

I go to the library every three days.
I go to the library once every three days.

In both those cases, we know the frequency -- one time within every three-day period, or three times in nine days, or ten times in thirty days, and so on. I would also understand it to mean that you always went on day 3.

Oops! I made a mistake in post #3.
I said 'Once in three days' isn't the same as "I go to the library every three days." 'Once in three days' means that you always go on day 3.
But I should have said: 'Once every three days' means you always go on day 3.​
If that is what confused you, I apologize.

Thank you for your replies.

Would you please explain why once three days is wrong? Because I think we can say once a week or once a month, right?
Do you mean I can't say once two weeks or once two months, either?

Do you mean I can't say once two weeks or once two months, either?
You can't say "once two weeks" or "once two months". "x times a <time period>" is idiomatic. It doesn't work if you change "a" to a number.
Once a second, twice a minute, three times an hour, four times a day, ...
Once two seconds, twice three minutes, three times four hours, ...

You can't say "once two weeks" or "once two months". "x times a <time period>" is idiomatic. It doesn't work if you change "a" to a number.
Once a second, twice a minute, three times an hour, four times a day, ...
Once two seconds, twice three minutes, three times four hours, ...
Thank you for your explanation.

Then if I do want to say "once two weeks" or "once two months, etc." I need to use in between once and the number + week/month/year, etc. as Cagey in #3 said. Is that correct?

once in two weeks, once in three months, three times in five years, etc.

once in two weeks, once in three months, three times in five years, etc.
This sounds odd to me. Be careful to avoid confusing these two meanings:

(1) one two-week-long period of time
(2) two one-week-long periods of time ("weeks")

You need to express these differently. You also need to express these differently:

(A) one single two-week-long period of time
(B) many repeated two-week-long periods of time

In my dialect I would say "once every two weeks" to express (1)(B). The "every" expresses (B), while "in" does not.

Of course there are other ways to express (1) and (B) in a complete sentence.

Thank you for your replies.

Would you please explain why once three days is wrong? Because I think we can say once a week or once a month, right?
Do you mean I can't say once two weeks or once two months, either?
You can say 'once a week' or 'once a month', but not 'once week' or 'once month', so you can't say 'once two weeks', but you could say 'once a two weeks' I guess. I hope that explains what is missing, however it would sound more usual to say 'once a fortnight', 'once in two weeks', once two weekly', or 'once every two weeks'. 'Once a two weeks' sounds archaic and just isn't commonly used.

. 'Once a two weeks' sounds archaic and just isn't commonly used.

In the US I have never heard anyone say 'once a two weeks.'

I have also never heard anyone in the US say "once two weekly."
"once a fortnight" I've heard in British English. I don't think it's used in US English.
I personally wouldn't say "once in two weeks." I would say "once every two weeks."

I have never heard anyone say 'once two weekly' either (I missed that earlier). I've also never heard 'once a fortnight' (I don't recall hearing the word 'fortnight' in any context) in US English. Fortnite, the online video game, is another matter.

PS: Americans who read British novels and watch British TV shows know the word 'fortnight,' but as far as I can tell it hasn't entered their English.

Thank you all for your replies. I appreciate it.

But I'm still very very confused.
What I know now is that "x times a <time period>" is idiomatic, such as once a week , twice a month , three times a year , etc. as long as it is a.

However, if I change a to other numbers, it will be wrong, such as once two weeks , once three months , once six years .

So how can I make it right? For example:
My doctor wants me to go back to the hospital once three months.

Can I say "My doctor wants me to go back to the hospital every three months" or "My doctor wants me to go back to the hospital once in three months"?

Is there a fixed pattern when the number is not a?

Other examples:
I usually go to the library every three days. or?
I usually go to the library once in three days. or?

you could say 'once a two weeks' I guess.
Once a week
Once a fortnight
Once a month

Once a two weeks

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My doctor wants me to go back to the hospital once three months.

Can I say "My doctor wants me to go back to the hospital every three months" or "My doctor wants me to go back to the hospital once in three months"?
Every six months (or some other period of time, like two weeks or three days) and once in the next six months (or some other period of time) are standard expressions in American English, but they don't mean the same thing.

In real life, my doctor has told me "get a blood test every six months." If I had gotten one today, he would mean "get a test six months from now, twelve months from now, eighteen months from now, and so on."

"Once in the next three months," though, means "any time in the next three months." "My doctor wants me to go back to the hospital once in the next six months" means that it doesn't matter when in the next six months I go back: I could go tomorrow, a month from now, or five months from now, so long as at the end of six months I've been to the hospital once.

Is there a fixed pattern when the number is not a?
Yes, just add "every":

once a week, once a month, once a year
once every two/seven/49/214 weeks, once every two/seven/49/214 months, once every two/seven/49/214 years

No need to be
very very confused

In the US I have never heard anyone say 'once a two weeks.'
As I said, it isn't commonly used in the UK either. It sounds either archaic (something you might find in Shakespearean period literature, or used by someone mimicking period English), or old fashioned regional dialect (I had an old grand uncle, a farmer in Somerset, who came out with things like that). I wouldn't use it myself but I would understand it.

Thank you all for your replies. I appreciate it.

But I'm still very very confused.
What I know now is that "x times a <time period>" is idiomatic, such as once a week , twice a month , three times a year , etc. as long as it is a.

However, if I change a to other numbers, it will be wrong, such as once two weeks , once three months , once six years .

So how can I make it right? For example:
My doctor wants me to go back to the hospital once three months.

Can I say "My doctor wants me to go back to the hospital every three months" or "My doctor wants me to go back to the hospital once in three months"?

Is there a fixed pattern when the number is not a?

Other examples:
I usually go to the library every three days. or?
I usually go to the library once in three days. or?
In once a week, twice a month, three times a year, you cannot 'change a to other numbers', because the a is not a number.
I think you are assuming it is functioning as a 'one', but it is functioning in this context more as something like 'per'.

Thank you everyone for your explanations.

once a week, once a month, once a year
once every two/seven/49/214 weeks, once every two/seven/49/214 months, once every two/seven/49/214 years
This is good!
I think you are assuming it is functioning as a 'one', but it is functioning in this context more as something like 'per'.
Yes! That's it! I think that's the problem that confused me! I regarded "a" as "one," so I didn't get it why it couldn't be replaced by other "numbers."

Thank you everyone for your explanations.

This is good!

Yes! That's it! I think that's the problem that confused me! I regarded "a" as "one," so I didn't get it why it couldn't be replaced by other "numbers."
Yes. Its natural to a native speaker so I never thought about it myself, but its probably a lazy contraction of something like 'each' or 'of the' as in the o' in o'clock.