When is Diego's school's anniversary?
1. It will be next month.
2. It is next month.
(Which answer is correct? Are both correct? Which one is commonly used?)
Both are correct. The second of the two is more commonly used, but MOST commonly used is simply, "Next month."
=)
Both are correct. It is already a date on the calendar, but the event will be celebrated in the future.
I would say 1. is somewhat better, because 2. makes it sound like the entire month is the school's anniversary.
I agree that both are correct. I would choose number 2 just because the question asks, "When IS...."
Still, both are correct.