What is the antonym for when?

There isn't a single perfect antonym for "when," as the meaning of "when" can vary depending on the context. However, depending on the context, you could use words like:

* Where: If you're talking about location instead of time.

* How: If you're asking about the manner or method.

* Why: If you're asking about the reason or purpose.

* Who: If you're asking about the person or entity involved.

* What: If you're asking for a description or specification.

Example:

* When: "When will you arrive?"

* Where: "Where will you arrive?"

* How: "How will you arrive?"

Ultimately, the best antonym for "when" depends on the specific meaning you want to convey.