I know that it's right, but can someone teach me how to explain this to someone? Thank you in advance.

All of us failing reading comprehension by missing the DOES NOT in all caps. :D

If you're writing a relative clause that relates to time, you need to include the preposition "on" if you use "which" as the relative pronoun. Same theory as using "in which" instead of "where" to refer to a place. Don't know why or if this rule has any sort of name you can look up. Just how it is.

Because you can’t be “off a day”. You can be “off ON a day” or “off WHEN”. That’s what I would say. I don’t know if it qualifies as a real explanation though.

It's not right. "on which" is the most right here.

To everyone who says "which" is not the correct answer. Read again, and more attention to the words written in capital letters :D.

OP knows that the answer to the question is right, and the question is in the negative. Reading comprehension ! Not a linguistic expert, but definitely “when” works since “a day” (one day or another) is a time interval. If you use “which,” the preposition “on” is necessary so we know that the description “[a time when] everyone is off” belongs to “a day,”or specifically falls on that day. If you omit “on,” it sounds like “a day” is literally being equated “everyone is off” which isn’t an adjective phrase. Honestly, I think most English speakers would understand all three. You could even use “where” or “that.” I don’t know how grammatical it is but for such a basic sentence/situation the context is easy enough to grasp.

I have heard native speakers say all three of these. I've also heard "where", "that", "in which", and "" (no word in the blank) among other constructions. None of them are very natural or good. Maybe a better grammarian than I am can say why one is more correct but in common usage they are all equally understandable but awkward. I would choose instead something like "The barbecue has to be on a day everyone has off" The barbecue isn't a day, its an event that occurs ON a day. And the phrase "everyone has off" sounds better without a "that" or "which" helping it.

Weird. I would use 'when', so I guess I've been saying it wrong my whole life lol.

It’s not right, as far as I can see. It could be if the verb were “has” instead of “is”. Otherwise the other two options are better (though neither is very good).

I'd say when. If it were date instead of day, I'd say on which.

Did YOU see the ALL CAPS before responding?

My God y'all 😭 I feel like I should've specified more in the title but anyways I just wanted to know ***how to teach*** other people on why "which" is the wrong option here 😞 sorry!

Funny cuz you could say "which everyone has off" and that'd work

"The barbeque has to be a day" Feels wrong to me, in of itself, regardless of what comes after. I would say "on a day." And, if it's "on a day," then "on which" feels awkward right after. I would use "when" or "where."

Ohhhh man I need to learn English

One helpful hint is that which very frequently is preceded by a comma. This doesn't help you if you are German, which for some reason puts commas where there are no actual speech pauses. In comparison, English is a language that limits commas to pausal situations.

"is off" (meaning not working) needs a preposition to link to a day. We have to use "on", so we know that we're talking about a time - not a day *which* is sunny. "On" is the correct preposition for a day. > I am off on Tuesday. Some people, informally, say "I'm off Tuesday" - but that isn't grammatically correct. For the same reason, "A day on which everyone is off" is correct; "A day which everyone is off" is not. "When" doesn't need a preposition here. "When" is *already* about time; it's a replacement for "on which". --- (Some people in the replies didn't read the question properly.)

I know this isn't the OP's question, but the sentence is a bit awkward. Generally, someone would say "The barbecue has to be on a day when everyone is off." You'll also hear "The barbecue has to be on a day that everyone is off" and The barbecue has to be on a day everyone is off." But you won't hear "The barbecue has to be on a day *which* everyone is off." In this sort of context, *which* is generally used with nonrestrictive clauses and *that* is used with restrictive clauses (here, "everyone is off" is a restrictive clause). Here's a Grammarly post on the topic: [https://www.grammarly.com/blog/grammar/which-vs-that/](https://www.grammarly.com/blog/grammar/which-vs-that/)

Honesty no native English speaker would have a problem with "which" either-- even if it is the "least" correct

First, I think it's cute that a Q emphasizing the difference between "which" and "on which" includes the phrase "the barbecue has to be a day" (which should really be "has to be ON a day").  Anyway, "which" is forward-pointing; "on which" is backwards-pointing. So "this is the table which I eat" means you eat the table; while "on which I eat" means you eat on the table. 

The question itself is wrong. English speakers say > “The barbecue has to be a day which everyone is off.” all the time

In reality, If you said "A day which everyone is off", it's not grammatically correct, but everyone would understand you. It just sounds a bit awkward. The word, "which" is usually not used to describe times or dates. We use word "when" for that. So if you wanted to use the word "which" here, you would want to use the additional conjoiner "on" to reinforce that you mean "ON that day."

Isn't "when" wrong as well because I feel like the sentence should be "The barbecue has to be ON a day when everyone is off". It doesn't really work without "on", no?

None of that is written correctly. Are they saying everybody should have the day off because there's a BBQ, or are they saying they should have the BBQ on a day when everybody is off?

honestly I feel like which is the only one that doesn't work

Both are correct. 'When' is more commonly used in everyday English, though.

(not native) I'd choose "when", "which" doesn't even seem to be correct for me.