There are a few options here:
- When is the end of the world?
- When the end of the world happens?
- When is the end of the world going?
- When does the end of the world happen?
Any wrong options here? Maybe you have had some other variant?
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This is grammatically and idiomatically fine. It's standard English. Another way of saying it is:
are both ungrammatical.
is a special kind of question. It implies a blasé attitude, sounds ironic, and suggests as much interest as "When does the shoe store close?" It's not a normal way of asking this question. But it's perfect for asking someone who believes that the apocalypse will occur today, for example, when it's going to happen. It's mockery.