I would ask the question: "What IS the day after tomorrow?" Since the day of the week (Monday, Friday etc.) or the date in the month (4th, 15th etc.) is already established, I wouldn't say, "will be". It is already recorded as the 10th or as Monday. The reply to this question would be, "It IS Monday" or "It is the 10th of September". I hope this helps

What will the day after tomorrow be? The reason: Question word + modal + subject + verb? What can you do? Where will you go? What will (the day after tomorrow - that's the subject) be? Actually, if you are asking what DAY or DATE it will be, we would say: What is the day after tomorrow? (Saturday) What'll the date be two days from now? OR What'll the date two days from tomorrow be? (Sept 7)

I believe the grammatically proper way is number 1 i.e. "what will the answer be?" as opposed to "what will be the answer".