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Context:
今日は高校の時の友達とランチに行って、少し買い物をした後、ケーキを買って私の家で食べました。...秋なので薔薇を飾ってモンブランを食べました。
The grammar book says that when "ので" is attached to a noun, you have to add "な" to make it a "Ｎな+ので" construction.
May I ask why the "な" is needed here? Is it to change "秋" from a noun to an adjective?
Copula form な developed as reduction of copula form なる during Late Middle Japanese (not to be mistaken with 成{な}る and 鳴{な}る), which developed earlier as contraction of にある (copula form に + existential verb ある).
This にある / なる / な could have been used with all types of nominals, i.e. nouns and nominal adjectives (most of them in modern Japanese form a group often called na-adjectives). (Around 16th century な was used both attributively and predicatively.)
In some usages, にある / なる was gradually replaced by である, and its contractions ぢゃ (surviving in parts of western Japan and spelt じゃ since 1946) and だ.
Somewhen during Modern Japanese, copula な generally stopped being used with nouns, but this usage survived in NOUN + なの(だ|に|で|...) constructions. Middle の in these constructions is nominalizer (which according to some linguists developed ultimately from case particle).
There is also copula form の (used only attributively). Synchronically more regular construction in modern Japanese would be NOUN + のの(だ|に|で|...) (where first の is copula and second の is nominalizer), but this seems rarely used.
See here for commentary about であるのだ, なのである, であるのである.
秋なの does not change 秋 from noun to adjective.
First, do you understand what's being said in 秋なので?
The meaning is "because it's autumn".
If you parse the parts, 秋 is "autumn". ので is "because". The other answers seem to dig deep into the derivation of ので or how な got to be what it is. That isn't necessary to understand what's going on here.
Just as you don't say, "because autumn" as a complete phrase, you can't say 秋ので. It's ungrammatical. You need the copula for "it's" in there.
In plain form, this is だ. You could say, 秋だから. That would mean "because it's autumn". But when you use ので, you need to change だ to な. The other answers dig a bit into the why of this.
May I ask why the "な" is needed here? Is it to change "秋" from a noun to an adjective?
Sort of, depending on what analytical theory you subscribe to and in particular what "adjective" means in that theory.
The の of ので is a nominalizer - i.e., a pronoun that exists specifically for the purpose of being described. This description is done in the usual way, by putting some attributive phrase in front. That could range anywhere from a plain verb or i-adjective to much more complex constructions.
な is simply the attributive form of だ・です - in modern Japanese, ordinary verbs that appear before a noun (in order to describe it) use the same form as they would as the main verb of a sentence, but the copula doesn't. (In special circumstances, である can also be used attributively.) So 秋だ "it is autumn" becomes 秋な "which is autumn", used adjectivally.
Sort of. This is where it gets nuanced.
Generally, nouns can be used before terminal だ・です without a problem. However, only certain nouns can normally appear before な. E-J dictionaries will typically use the term "na-adjective", but this properly should mean the combination of the noun with な. (Actually, some words only work this way and don't fill all the normal roles of nouns. For example [綺麗]{きれい} doesn't work before が, and needs to be adapted in order to talk about "prettiness" abstractly.)
So, we might think of using the other usual way to describe nouns: by classifying/categorizing them (or marking possession etc. etc.) using a noun + の. But then we run into another problem: we would have two のs in a row, serving totally different purposes.
In practice, this isn't done - instead, な is used to connect to the の nominalizer, even if the noun preceding な wouldn't normally fit there.
なので can be thought of as な+ので, but I think it is better understood as なの+で, treating なの as a noun suffix meaning "-sort of thing". The combination なの, with this effect, is also seen in some other contexts.
