"Today is Sunday."
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/
Eventhough many English speakers tend to overly use the 是 when speaking, I do think in this case it should have been used and accepted. For those of you who are interested, check out the youtube channel "Everyday Chinese". There is also an accompanying website with loads of free lessons to download and it has helped me immensely since I started studying Mandarin about three years ago. I use Duolingo and that channel exclusively as supplemental learning tools. Good luck everyone!