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What's idiomatic of these two choices ("In what month" Vs. "In which month")?

I were told in the past that I have to choose "what" in a place where I don't know anything about the choices (for example: "On what floor are you?") and "which" in a place where I know about the a specific choices out of others that exist ("On which floor are you, first or fourth"?). Assuming that it is true, I am asking about months if it's the same, because I see not native English speakers that use both interchangeably especially about months.

"In what month do they celebrate this holiday?"

"In what month were you born?"

Vs.

"In which month do they celebrate this holiday?"

"In which month were you born"?

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  • It should be do they celebrate, and while were you born is correct the first time you use it, you have omitted you the second time.
    – Jason Bassford
    Commented Dec 12, 2018 at 20:52
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    Both which and what can be used in your sentences. It's simply a personal preference. (So, yes, it's the same for months.)
    – Jason Bassford
    Commented Dec 12, 2018 at 20:54

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What you were told is incorrect. Rather, "what" is always idiomatic, but "which" is only correct when you have a fixed, usually small, set of options. When "which" is correct, it is slightly more strictly so than "what".

For your examples: "which" is usually acceptable for floors, even if you as the asker don't know how many there are, because you do know that there are a limited number and that for most buildings that number is going to be small. It is acceptable for months, too, because while the number and names of the months may vary depending which calendar you're using (a fact that's particularly important when looking at holiday celebrations!), again there is a fixed set of options, usually twelve or thirteen. As a native speaker, I would normally use "which" for months, but I know plenty of other native speakers who would tend to use "what". In a formal context, "which" would be clearly the better choice, but in an informal one it really doesn't matter.

"Which" would not be correct for an open-ended question. You can ask "what do you want?" without providing nearly as much context as you'd need for "which do you want?", for example.

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    Which has nothing to do with a limited number (or fixed amount) of anything. I could easily say In which of the world's millions of cities were you born? and it would be perfectly fine to do so.
    – Jason Bassford
    Commented Dec 12, 2018 at 20:56
  • @JasonBassford Which absolutely has to do with limited sets of choices. "What" is always acceptable, but "which" is not, and the cases where "which" is not acceptable are those where the set of possible answers is open. The degree to which people prefer "which" when the set is closed is, in my experience, inversely proportional to the size of the answer-set. You had me questioning myself for a moment, but see also the same question on ELU
    – Darael
    Commented Dec 12, 2018 at 20:59

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