@Dondic, we do not use ”day” for that, but refer to ”time” (like in ”it's my time”), or a more precise period, like ”hours”. For example, in the evening when the child doesn't want to go to sleep and keeps jumping around and playing silly just to avoid going to bed, some family member who want to hold his/her side may say „Lasă-l/Las-o în pace (/rabdă-l/rabd-o/), e ora lui/ei acum” (”Leave him/her alone/endure him/her, it is his/hers hour/time now”) (especially if he does that every evening). This is used also with the meaning ”it is his/her turn”, albeit we have an expression for that too („este rândul lui/ei”).
The beautifulness of Romanian language is that the speaker can be as clear or as vague as he wants, and we have many jokes, proverbs, and popular wisdom words capitalizing on that.
I tried with ”will be” and it was rejected. I had the feeling that English speakers never talk about the future at present tense. In the Ge from En course they insisted on the fact that in German, contrary to English you can talk about the future at present tense. That is why many Germans will say in English, for example ”I come tomorrow” instead of ”I will come tomorrow”. This is especially stressed in Michel Thomas' English/German course, where he dedicated a full chapter to it. He says that Germans make this mistake even if they have a very good command of English, because their speech pattern is in the present (”Next week it rains”, incorrect English, but correct German), and I remember that thing especially because in Romanian, same as in German, you can talk about the future at present tense. We say ”Vin mâine” with the meaning ”Voi veni mâine”. And when I was listening to Michel Thomas (which I do everyday in my car on the way to work and back, more than an hour totally daily - I am trying without much success to learn German for years - I think I am too stupid for that language, even here on Duo, where the course is very well done, I repeatedly get stuck, erase the progress, start from scratch, etc), so when listening to that chapter of the audio book, I was always thinking ”wow, I have to be careful not to make this mistake in English”... ”Săptămâna viitoare plouă”. As Michel Thomas would say, ”correct Romanian, incorrect English”....
So, asking a native speaker, is this translation given by Duo, right in English?
Yes, we do say things like, 'tomorrow it's my birthday' or 'next week it's my birthday'. We also say things like 'He's going on holiday tomorrow' or 'We're having a party on Friday.' However, I don't think I've ever heard anyone say, 'tomorrow it's raining' or 'it's raining next week'. We would always say 'tomorrow it's going to rain' or 'it's going to rain next week.' Why the difference? I don't know. I only speak the language ;-) However, there is a more detailed explanation here: http://learnenglish.britishcouncil.org/en/english-grammar/verbs/talking-about-future
poimaine === "the day after tomorrow"