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.

Both answers are grammatically correct, but the second answer "It is next month" is more commonly used to state the date of an event or anniversary. This answer directly specifies that Diego's school's anniversary falls in the upcoming month. On the other hand, the first answer "It will be next month" is also correct and conveys the same meaning, but it adds an element of future tense to indicate that the event will happen in the future.