Translation:What is tomorrow's date?
Right. The 号 basically indicates the question is looking for a number in the answer, rather than the name of the weekday, which is almost certainly why the default English version of the question says "day of the month" rather than simply "day" or "date" (albeit, "date" is slightly better than "day," here, but still not the best choice for teaching this particular word, given the broader usage of "date" in English).
I think the issue is 明天几号 might be asking about the date as apposed to the day of the week. I think you would ask the day of the week with: 明天星期几？In my dialect of English (American, grew up near DC) I think I might ask both of these questions as "what day is tomorrow?" and you would have to know from context which I meant.
That is correct. In conversation with other Chinese people, I would express it as hòu.
I put "what is the date tomorrow" and it marked me wrong. :( i think this means the same thing despite being slightly different phrasing and is not wrong. Im british and i dont think i would ever hear people ask "what day of the month is it today", but they would ask "what is the date today" if they forgot what day of the month.
I agree that the translation, "what day of the month is it today" sounds somewhat artificial in (either British or American) English, if only because "what is the date tomorrow" is less wordy, and, I, too, personally use "date" as you do, to mean "numerical date." However, "date" can also mean "full date," e.g., Mon 18 Mar 2019, and, other English speakers typically say "date" when they mean "day of the week." So, consider the options that that range of possibilities presents the developers of this course. Yes, the course developers could allow a greater variety of answers, but then some respondents would get a correct score without really understanding the meaning of the Chinese (e.g., someone who uses "date" to mean "day of the week" might try "what's tomorrow's date" and then get a correct score, whereupon that student would be left with the wrong idea of what 几号 really means). So, rather than let some students "slip through" with the wrong impression, the course developers ostensibly opted to phrase the correct translation in a way that expresses more literally what the Chinese phrase actually means, albeit, at the expense of good "flow" in English prose.
In sum, the point of the exercise is to teach the (precise, correct) meaning of the Chinese, rather than to teach natural sounding conversational English. The result is that English speakers must respond in a way that (especially to native English speakers) sounds a little "off" or verbose, but the trade off is that by providing that (admittedly awkward) answer, we are indeed conveying the precision of what the Chinese means, and to that extent concomitantly learning "to think in Chinese." If the answer we must provide (the "password," if you will) is "what is the day of the month tomorrow," then no one will pass this exercise without learning that 几号 actually means "numerical day of the month," and that bit of information is the very point of this exercise, after all.
"Tomorrow is what date?" should be correct. Duolingo needs to review acceptable answers.