It's a day when we all come together and donate money to try and stop cancer.
It's a day where we all come together and donate money to try and stop cancer.
How would you teach English learners that the second one works fine?
Both sentences are okay. The "where" in the second one is idiomatic and as such does not need to make strict grammatical sense.
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Both sentences are okay. The "where" in the second one is idiomatic and as such does not need to make strict grammatical sense.