| The immediate answer, which perhaps not everyone knows, is that Shir Hashirim is the book of the Tanach that we read on Pesach, specifically on the Shabbat that falls during Pesach (some complications omitted here).
So we go on to ask: Why is Shir Hashirim read on Pesach?
It's obvious why we read Esther on Purim and Lamentations on Tisha B'Av---it wouldn't make much sense to do it the other way 'round. But there's no such obvious connection between Pesach and the contents of Shir Hashirim.
To understand the answer to this question we have to ask an even more fundamental question: Why is Shir Hashirim part of the Tanach at all?
It doesn't tell a historical story, there's no prophecy or laws or moral teaching, the name of God doesn't even appear. It's just a bunch of love poetry, some of it bordering on the salacious. What is this near-pornography doing in the Bible? Indeed, many early sages thought it had no business being there. What gives?
The answer is that from earliest times the Song of Songs has been interpreted, not as an expression of human romance, but as an allegorical conversation between God and Israel.
The literal words of the book are simply Solomon's way of casting deep meanings into poetic and beautiful language. He brilliantly chose the metaphor of love, with all its ramifications---including sexuality---to explain and explore the various aspects of God's complex relationship with His chosen people.
|As an example, here is a literal translation of the first few verses (minus a few phrases):
||And here is the allegorical translation of exactly the same passage, following Rashi's interpretation:
|The Song of Songs, which is Solomon's.
Let him kiss me with the
kisses of his mouth, for thy love is better than wine.
have goodly fragrance, thy name is ointment poured forth, therefore
do young maidens love thee.
Draw me, we will run after thee. . . .
I am black, but beautiful, O daughters of Jerusalem!
||The song that excels all songs dedicated to God, the King to Whom
[Israel addresses God, saying:]
innermost wisdom to me in loving closeness, for your friendship is
dearer than all earthly delights.
Like the scent of goodly oils is
the spreading fame of Your great deeds; Your very name is Flowing
Oil, therefore have nations loved You.
Upon perceiving a mere hint
that You wished to draw me, we rushed with perfect faith after You
into the wilderness. . . .
Though I am black with sin, I am comely
with virtue, O nations who are destined to ascend to Jerusalem!
Get the idea?
This understanding of the Song of Songs is very old. It goes back at least to the first century BCE.
Rabbi Akiva himself argued strongly that the allegory was the only way to interpret the book; his famous words are "All the Writings are holy, but the Song of Songs is the Holy of Holies." And that's how it comes to be in the Bible.
Now we see the connection between Shir Hashirim and Pesach: Pesach is the holiday commemorating the awesome physical realization of the relationship between God and Israel: the creation of Israel as a people via the exodus from Egypt, an unprecedented event carried out by God personally. So of course the appropriate book to read is the poetic exposition of that same relationship.
Some disclaimers: I'm in no way a Torah scholar so you should take all of this with a grain of salt. Certainly there's lots I'm leaving out.
And I really should permit modern scholarship to tell its side of the story. For example, linguistic analysis makes it pretty clear that the Song was written by several different people, and that Solomon couldn't have been one of them!
But I've got charoset to make. So you all have a chag kasher v'sameach.