"Tomorrow is the 18th."
It depends: in conversaitions it's very common to be omitted but in written Mandarin, no.
It's frustrating that in previous example Dou marked it wrong if "shi" was omitted. But here they have has omitted it themselves.
I was marked wrong for using 日, am I wrong for thinking you can use both 日 and 号?
I think using 日 means the number of days. Like 五日 means 5 days.
As appose to 五号, which means the 5th day.
Just my guess.
This is incorrect, I'm afraid. 日 is formal and used in writing, and 号 is more informal, and used in conversation.
No, you're right. You can use either 日 or 号. The difference is: 日 is considered more formal and is mostly used in writing, whereas 号 is less formal and used conversationally. (You virtually never see 号 used on calendars.)
Spoken: 几月几号？ Witten: 几月几日？