"What festival is it tomorrow?"
Keep to 明天是什么节日。
明天，节日是什么 is a wrong structure.
明天节日是什么 is a wrong structure.
However, 明天的节日是什么 is weird because it sounds as if the day possesses the festival =_= In English you could probably say "What is tomorrow's festival", but not in Chinese.
Yes, I agree with all of the above. Had said the sentence aloud, I might have picked up the weirdness of it, and adding 的 made it worse... 哈哈哈！
In that structure, adding 的 was necessary, it made the sentence weird, but not adding 的 is just completely wrong grammatically...