"Tomorrow is the 18th."
I noticed many commenters saying they included "是" in their answers and it was marked wrong. Duolingo didn't even have that as an option in my answer box. Just a little bit of input my Chinese teacher gave me, she's obviously a native speaker, from Beijing. She said that English speakers tend to overuse "是" as a connecting word, so you need to throw out your "English" way of thinking/speaking. Spoken Mandarin and written Mandarin will many times have different rules and this is one of them. In spoken Mandarin, you can omit the 是，in written form, you would most likely include it. However, there will be many times where it will sound as if you should use "是", but it is not needed nor is it proper in some cases. Just remember, it's not English. Check out the youtube channel "Everyday Chinese". Aside from my classes, it has helped tremendously. My 老师 recommended it, and between that channel and Duolingo, I have progressed immensely. I started to progress when my mindset changed- I used to think that Mandarin was like English and the same rules applied, some do, many do not. Once I got over that, things came more naturally. Good luck!
It seems that Duolingo puts bu shi if it's negative, but not shi if it's positive. In spoken Chinese, is the verb less likely to be omitted in a negative sentence? Or is it acceptable to say, for example, "ming tian bu xing qi tian" (sorry, I don't know how to put the characters in)