〜なので: because (it) is … (after a noun or na-adjective)

Noun / na-adjective + なので = “because it is …”. だ turns into な before ので.

ので follows a plain form. With verbs and い-adjectives nothing changes (行くので, 高いので). A noun or na-adjective ends in だ in the plain form, and だ becomes な before ので: 雨だ → 雨なので, 静かだ → 静かなので, 初めてだ → 初めてなので (“because it is the first time”). This な is the same one as in 〜なのに and 〜なんです. It is not the な that joins a na-adjective to a noun (静かな部屋). Compare から, where だ stays: 雨だから. Very polite speech also uses 〜ですので.