# “which day is Friday” vs. “what day is Friday”

Thanks to: "Which" vs. "what" — what's the difference and when should you use one or the other? I know that "what" can replace "which" in the examples below. But which questions sound more more common?

• Which/what day is Friday? — It's the fifteenth.
• Which/what day is the first of November? Is it Saturday?
• Which/what month is the warmest in your city? — It's July.
• Which/what season is it now in your city? — It's summer.
• Is it a choice between things? Which. Is it a definition or can be answered in different ways? What. Although in parlance substituting what in place of which is understood, the reverse will on occasion sound awkward, especially given the criteria in the first sentence of this comment. – SrJoven Nov 2 '14 at 11:25

For this, I have to turn to Google Ngrams:

What/Which day is

What/Which month is

What/Which year is

Based on the Ngram results, it seems clear that what is far more common today for all questions relating to dates (however, that wasn't always the case). This is also consistent with my preferred usage and experience, where I would hear what used probably 9/10 times.

What day is Friday?

There are 30/31 days in every month (except for February). That's quite a lot of numbers to choose or guess from. Faced with such an open-end question, I would first reply

A) Which Friday are you talking about?
B) Next week's
A) Erm, let me think. It must be 21st

A) What day is it today?
B) It's Tuesday

If today were November 11, and someone wanted to know what day of the week is January 1st he or she might ask

What day of the week is January 1st?
or
What day is New Year's Day?

There's only seven days in the week to choose from, that's not very many. The person replying could make a wild stab and guess correctly. But your average Joe will have to calculate, and maybe think back to which day of the week the holiday fell on last year. If I wanted to know, I'd need to check my calendar.

Now, imagine today is the 27th December. That same person might ask me (it's not a guarantee, but it's possible)

Which day of the week is January 1st?
or
Which day is New Year's Day?

There's only four days left to the New Year, even I could answer that question easily. Which in the above examples is being used as a determiner or sometimes called an interrogative adjective.

If I wanted to know what month you were born, I'd ask

What month were you born?
OR/AND

We know that the new Star Wars film is going to be released in 2015, but we want to know its date of release.

When is the new Star Wars movie coming out?
When is the new Star Wars movie being released?
When will the new Star Wars come out?

It's coming out December 18, 2015 (USA)

We wouldn't normally say: What month is the new Star Wars coming out? It's more natural to ask when, rather than specifically asking which month of the year.

If you're asking about the hottest month in a country, I'd suggest that you say

A) What's the hottest month of the year in [country]?

B) June is usually slightly wetter than July & August and hence cooler. In some parts of Asia, summer is the wet season (monsoon season). In this part of the world the hottest month is the last dry month of the year – usually April or May.

To me the more common sounding are: What day is Friday? What day is the first of November? Which month is the warmest in your city? Which season is it now in your city?

I could ask " Which of the months is the warmest in your city?" and "Which of the seasons is it now in your city? and expect the same answers.

"Which of the days is Friday?" or "Which of the days is the first of November?" are questions that aren't clearly defined, so 'which' sounds wrong for those situations.

Having given my answer, I would also like to make a comment, but I don't have enough reputation for that so I will add it here.

The question asked is "But which questions sound more common?" (my italics - and my dropping of the duplicate "more" which can't be edited out as it isn't six letters).

This is a question that can only be answered subjectively. In each case the sentence construction that sounds more common to me, an Australian, may not be the sentence construction that sounds more common to, say, an American.

Kip's answer is similar to the one we often see. Michael Swan, for example, (Practical English Usage, 2005.611) writes:

We prefer which when we have a limited number of choices in mind.

We've got white or brown bread. Which will you have?

(More natural than ... What will you have?)

Which size do you want - small medium or large?

.

When we are not thinking of a limited number of choices, what is preferred.

What language do they speak in Greenland?

(More natural than Which language ...)

.

That seems moderately clear, but note that Swan uses the word preferred, not mandatory.

Huddelston & Pullum (The Cambridge Grammar of the English Language, 2002.903-4) note a similar difference in usage to that noted by Swan, but also say that in some cases, "The difference between what and which is effectively neutralised".

Now, let's look at your original questions:

Which/what day is Friday? — It's the fifteenth.

Which/what day is the first of November? Is it Saturday?

Which/what month is the warmest in your city? — It's July.

Which/what season is it now in your city? — It's summer.

As Huddleston & Pullum say of different sentences, "... which encodes the fact that the choice is from an identifiable set" [thirty-one dates, seven days, etc] "while what doesn't. but as that is part of background knowledge it doesn't matter from a pragmatic point of view whether it is encoded or not".

My own somewhat cynical view is that grammarians have tried to make a usage difference which many native speakers do not feel. Some native speakers use which if there is a very clear limit to the number of choices, and what if there is apparently no limit. Others use whichever/whatever wod comes to mind first. Very few people will be upset if you make the 'wrong choice. Most won't even notice.

In your sentences, I'd use what in the first two (I think) and either word in the third and fourth. Others might well make a different choice. Sadly, I don't think the bonus is going to attract a definitive answer.

This question is poor, because "what" and "which" are words which are used differently (and sometimes it matters).

Take my sentence above, for example. It must read:

• "words which are used differently", and it cannot be
• "words what are used differently"

"Which" can be used interrogatively, in which case it seems to be always interchangeable with "What".

"Which" can also be used as a relative pronoun, in which case I don't think it's ever interchangeable with "What".

To be clear, "What" can also be used as a relative pronoun, but you just can't ever substitute it for "Which".

Also, both words are not limited to usage in these two ways, but these are most relevant to your question.

• This doesn't really answer the question. Peter is asking about a preferred usage give a limited set of questions, not the difference between "what" and "which." And based on his question, it seems that he is already clear what the difference is. – Nick2253 Nov 10 '14 at 18:05

If you want to know the word that is used most often in a question, we can take a look at Google using wildcards. You can copy and paste these searches for yourself if you like:

"What" * "?"

"Which" * "?"

Sentences or phrases starting with a capital W for both words and ending with a question mark with anything in-between is what both of those searches provide results for.

I know this isn't an answer with some sort of official citation, but popularity-wise, this is a good way to judge what is most common, which is what you requested in your question.

So the winner is... What

The way I read it is one would use which when there is a (limited)choice between a few defined possibilities, and what when there are more choices and/or when these choices are not so defined.

Used in a phrase I would say;

• Which of these Friday's would be best for you? Friday the 6th? Friday the 13th? Friday the 20th?
• What would be a good day for you?
• Hope this helps,

K!P