Seems to me that it's more natural to say "Ayer fue caliente". Of course, we already that yesterday was day, we said is much in the word "yesterday".
Calor is a noun which really means 'heat'. El fuego produce calor = Fire produces heat.
Calurosa is the feminine form of the adjective 'caluroso' meaning 'warm' or 'hot' and as I understand it is not actually used that much except in describing weather. Es un día caluroso = it is a hot day. Like Duo's given sentence.
Caliente is an adjective meaning 'hot' as in hot to the touch. El horno está caliente = the oven is hot. Though I have seen it used for 'spicy' as well. La pimienta está caliente = the pepper is hot.
Edit One caveat here: You could use 'calor' for weather as well however you would use it with the verb hacer. Hace calor afuera = it is hot outside.
Here's the thing. "Yesterday was very hot" is a perfectly fine sentence, but it is not the sentence we were given.
I think one of the problems we're all having with this translation is the repetition of "day" in the English version. It sounds less clunky in the original because "ayer" doesn't include "dia."
One of the problems with the Duo method is that we are taught new idiosyncrasies (past tense ir vs estar having interchangeable application) as well as new words, verb forms etc all within the same lesson; typical with this particular lesson. Older folk like me whose grey matter struggles with multitasking can take a while to absorb it all, whereas the more photographic memories of the younger generation can lap it up!
I totally agree!
An even bigger problem for me is when Duo introduces all these idiosyncrasies together, in combination, I can't separate them out so I don't understand the individual applications of each (what may or may not need to go together) and I find it difficult to apply them elsewhere. There's no focus on just one at a time and the tips don't give enough of an explanation. I find these forums invaluable for that comprehension.
I suppose at one point we get far enough along in the lessons to where Duo hopes that we, the users, will/should pick it up after ad nauseum repetition (which sometimes only pounds in that sentence for me) and usage but I still get a little frustrated and confused. 😖