What is the useage of "な" in "~なので"?


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](https://en.wikipedia.org/wiki/Nominal_\(linguistics\)), 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](https://japanese.stackexchange.com/a/26253/) 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.**

