Dex, thanks for the good wishes, hope your Yom Tov was enjoyable as well.
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Well, the reference is to “THE young woman,” not just “A young woman”, so the implication is that it is someone specifically known to Isaiah and Ahaz. The source of my explanation is Rashi’s commentary, who seems to base it on the conception and birth, in the following chapter, of a son from Isaiah’s wife. He refers to her as “the prophetess” and so Rashi (probably based on an older Midrash) takes that to mean that she had a prophetic experience - i.e., naming the son in tune with the prior chapter’s prophecy. In the verses that follow the child’s birth, Isaiah addresses prophecy to a party known as “Emmanuel”, and while that seems (from the context) to be a poetic reference to the people of Judah generally (and not to a baby), it’s hard to imagine that he uses the name randomly. The idea behind the significance of the name Emmanuel in the prophecy is that it means “G-d is with us”, i.e., that the mother’s name for her son would be a prophetic omen for Judah overall. Thus, the fact that Isaiah has begun only now to address the people of Judah by that name would seem to indicate that a new factor - i.e., the birth of the prophetess’s child - has brought this about.
Whew. It’s pretty flimsy, I’ll admit, but that’s about par for the course for scriptural bases of Jewish oral tradition.