In the tips section for Weather 2 it says, "Remember when we told you that adjectives come before the noun? Well sit down friend, we need to have a talk…
There are certainly some adjectives in Gaelic that come before the noun, though the overwhelming majority come after. The ones we come across always lenite the noun that follows if they can..."
But I thought we already knew well before this skill that adjectives come after the noun. It says the majority of adjectives come after the noun, then proceeds to talk about "deagh" the adjective coming before "latha" the noun... Maybe I'm just very confused.
It could be both here, they mean basically the same thing.
Although it is worth to note that as a greeting (hello, good afternoon!) you would only use feasgar math, as deagh fheasgar doesn’t seem to be used this way.
deagh fheasgar might emphasise the quality a bit more, like is it really a good afternoon? when feasgar math could be interpreted more like an OK afternoon, not particularly bad one. But also both can mean exactly the same thing and you shouldn’t worry about the possible slight difference in shade of meaning.
In general math and deagh are interchangeable when attributing nouns; in some contexts one or the other is more popular (but typically both are possible nonetheless). The slight differences between them is something a learner IMO shouldn’t worry about too much, you’ll learn it by immersing yourself in the language later.
But if you really want to dive deeper, there are two works by Veronika Csonka on them: the paper Subjectivity and emphasis in Scottish Gaelic preposed adjectives and her PhD thesis on preposed adjectives in Gaelic.