"Today is Saturday."
There are actually three ways to express the days of the week. 星期[#] (Xīngqī), 礼拜[#] (Lǐbài), and 周[#] (Zhōu). I was told 周 is never spoken and only used in print.
Not a glitch. Another way to express the day of a week. If you want to express its common form, then it is 星期六.
No, it's just one of our wonderful Duolingo Chinese team's tricks to shake off the rust from our brains and make us use some logic, maybe even consult a dictionary. :) If the translation we are already used to is not there, and all other options are definitely wrong except a new one which we've never seen before... It must be another new way to express the same thing.