Doesn't 星期日 also mean Sunday? Why is it not accepted as an answer?

周日 what the heck????

It accepted 今天星期天 for me, but does it sound weird to not include 是?

These characters for Sunday were not introduced, How do they sound! How many different combinations are there for Sunday!!

What's the difference between "星期日", "星期天" and"周日"?

I am having a difficulty knowing when to add 是 for“is” and when adding it is wrong. I usually think it and then have to drop it for the exercise. Then there is this one where it seems to be required. Any help or guidance on this?

if previous questions are anything to go by, "今天星期天" should be accepted.

No sound for Sunday will report 10-16-2018

今天是星期天 not accepted. That's definitely a mistake.

Why is ”今天星期日” not an acceptable answer, also?

I found a site to help but it still bothered my that they threw that word in without previously teaching it. https://blog.speechling.com/5-different-techniques-to-indicate-time-and-tense-in-chinese-speech-and-writing/

this set of characters is not available for the answer "today is sunday"

Totally confused why the answer without (shi) is accepted but it's not likewise in questions form

Surely the answer should be 今天是星期天？

Why my answer is not accepted? the answer is same