"It is a hot day."
Translation:On kuuma päivä.
I don't think there's anything to trigger the partitive here. In phrases like hyvää huomenta and hyvää päivää, someone said here that it's shortened from something like 'Toivotan sinulle hyvää huomenta', where partitive hyvää huomenta is the object of the sentence.
But the verb on doesn't take an object, just a subject and predicative. A singular countable subject or predicative would usually be nominative singular.
A couple exceptions would be in a negative sentences like 'Tuolla ei ole koiraa', and possessive questions like 'Onko sinulla koiraa?', but 'On kuuma päivä' is neither type of sentence.