"Is tomorrow Sunday?"
They mean the same, no difference. Actually, there are quite a few intetchangeable variations for Sunday: 星期天，星期日，礼拜天，礼拜日，周日 (but NO 周天, that would be incorrect). Pinyin is li3 bai4 for 礼拜，and zhou1 for 周.
They have exactly the same meaning. It's up to you which one you prefer to use at any given time.
Theyre the same but i think its more common to say 星期日 on the mainland and星期天 in taiwan
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.
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.
Often, 是 is omitted in spoken language. It is less formal but still acceptable.