"Is tomorrow Sunday?"
It's actually best (more proper/correct and common) to use 是, but it's allowable to omit it as well. Allowable, but not preferable. I double checked this answer with a native speaker before sharing, since I was confused at the omission as well. I disagree with the course authors omitting 是 in this instance.
By the way, if you also want to know something, then up-vote the question, or simply wait for someone to answer it, instead of writing a separate post letting everybody know this. These are "Me too"-like posts, which are not helpful to the discussion. This is a general rule applying to any internet forum. Discussions become tiresome to read if there are a lot of 'me-too'-posts. Of course this applies not only to questions, but also to statements, opinions, etc.