Either would be acceptable, but "What is the date" would be preferred in my not so humble opinion.

Saying "what is the date" asks for the calendar day when something is occurring.

Saying "when is the date" asks when an event, "the date" is occuring
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Since "date" is a noun and the subject of the question, the correct response is "What is the date..." To use when, the question would be "When are we going to lose this content?" or "When is this content set to expire?"

I would use "What is the date that..." to elicit an exact response, i.e. "This content expires on February 6th." The "when" question may elicit a more vague response, i.e. "This content expires in three days." Is that three days from now or three days from when I sent the email yesterday?
Source(s):
Basic English grammar knowledge.

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What is correct simply because it refers to the word date, which is a noun. Also, being a number denoting a day in the calendar, and not a 'moment' in time, it is more appropriate.
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