We read Shir Hashirim on Shabbat Chol Hamoed Pesach, or, if there is no Shabbat Chol Hamoed, on whichever of the last two days of Pesach falls out on Shabbat. Shir Hashirim is a very hard Megila to understand. Literally, it is a love song or serenade. If this is true, what is it doing as part of our holy scriptures?
Rashi, along with many other commentaries, concludes that Shir Hashirim is an allegory to the love between Hashem and Bnai Yisrael. King Shlomo laments over the separation between them that will come with Bnai Yisrael's exile from Israel, but he also foresees their return. This would explain why it is read on Pesach; Pesach is the time when we recall Bnai Yisrael's exile in Egypt as well as their escape by the hand of Hashem.
Although this interpretation of Shir Hashirim makes sense when looked at in the light of Pesach, it still seems odd that King Shlomo would write something that could be meant to connote something completely different. To take a Pasuk (1:2) as an example, a literal translation reads, "Oh, that he might kiss me with the kisses of his mouth, for your caresses are more pleasant than wine," whereas an allegorical translation says, "Communicate Your innermost wisdom to me again in loving closeness, for Your friendship is dearer than all earthly delights." The differences are obvious; how can there be two entirely different translations to the same Pasuk? Furthermore, the only other time Shir Hashirim is read is before Kabbalat Shabbat. There is a Minhag that on Shabbat night men read Shir Hashirim to their wives. Seemingly this is due to the fact that this is a love song, and it is recited at this time because of the loving atmosphere that is brought by Shabbat.
The similarities between these two interpretations, however, can lead to reconciliation regarding these differing understandings. The allegorical version refers specifically to the deep love between Hashem and Bnai Yisrael, whose love for each other are said to be like the love between a husband and wife. So long as this idea is still intact, the point of Shir Hashirim can still be expressed; it is only due to the function of Pesach that it is translated to draw out the aspect of Geula that can be taken from the Shir.
It is important that one understand the true meaning of Shir Hashirim. Even though the allegorical translation has meaning during Pesach, the literal translation is still important to understand. It is safe to assume that King Shlomo's writings can be taken at face value, even though one may be able to extract certain lessons from them, and so to replace the literal translation in favor of an allegorical one might deprive the reader of a true meaning of Shir Hashirim. (See, however, the first footnote of Rav Yosef Dov Soloveitchik's classic essay entitled "U'Vikashtem Misham".)