Answer :
Tomorrow is a holiday is right. At the same time tomorrow will be a holiday is also right. But you should definitely include the article 'a'
The correct answer is: "Tomorrow is a holiday."
Explanation:
The second sentence is defining certainty and the one who is giving the notice about the holiday must be providing it in present tense. While in the first sentence, "will" makes the information more like an oral conversation, not a notice. Therefore, it becomes very definite that right choice is "statement number second."