What I find more unusual is the idea of calendars in plural. Where I'm from a calendar is stuck on the wall. I can't imagine I would ever need more than one! Maybe some people collect them, green ones, then you could say, "all my calendars are green." Haha :)
Edit: Oh, oh! Perhaps this is someone who sells calendars! Green only! :)
What Duolingo aims for is not to teach you everyday-life sentences to learn by heart, but rather to make you understand single and individual concepts for you to become capable of using them in every way you feel like and combine them in the way you need them. Unsual (but grammatically correct) sentences do play an important role in that.
But they're different things - an almanac will always contain a calendar, but never vice versa. In fact, it was illegal to publish almanacs in Sweden unless you had royal permission - as late as in the 1970s! :)
That said, a lot of people obviously use the words interchangeably.
The confusion for some here (particularly Brits) is that Swedish uses the term calendar in the same way as American English, but not in the same way as British English. In British English, a diary is normally the book where you note down appointments and events on particular dates. A calendar is usually hung on the wall, or placed on a desk, and may or may not be used to write appointments on. Diary in British English can also mean a personal journal. I have had lots of green desk diaries :)
I'd argue there's significant overlap in British English as well, especially since the advent of ubiquitous calendaring software. But Swedish (somewhat clumsily) uses kalender for the stationery and wall sense as well as the notebook almanac sense, so there's lots of leeway when translating.
I might add that the "personal journal" sense of "diary" is dagbok (literally "daybook") or journal in Swedish.