What day is this?
Radak: It is the day when Tzor will be destroyed.
Why does it say "v'Nishkachas Tzor"?
Rashi: Because it is called a Zonah for disgrace, it says that it will be forgotten like a Zonah; her lovers forgot her. Also Tzor will forget from where its merchants turn to her, for it will be destroyed.
Radak: It was decreed that it will be forgotten, and not rebuilt for 70 years.
What is the comparison to "Melech Echad"?
Radak: Like the days of David ha'Melech 1 - 70 years. Radak - this is because David had a covenant with Chiram Melech Tzor. After David and Shlomo died, they did not guard the Bris, and harmed Yisrael as much as they could. Their kingship will cease for this time, so they will remember David's Bris with their king, and they did not guard it, and they will know that the decree was due to that sin.
Radak: Man's days are normally 70 years - "Yemei Shenoseinu Vahem Shiv'im Shanah" (Tehilim 90:10). For this time, you (Tzur) will cease from kingship. Because it was a great kingship and very honored, just like its king and kingship can live (70 years), so it will be without kingship.
Malbim: It is like the days of one kingship - Malchus Bavel was 70 years. In the days of Koresh, [Tzor] rose a little.
Rashi: I do not know why this sign was given (that its years will equal those of David).
What will Tzor have?
Rashi: It will have a small remembrance. It will be aroused from its lowliness. It will turn to those who used to go there to return to it, like it used to be.
What is the comparison to a harlot's song?
Rashi: She sees that men do not come to her, so she sings in a loud, pleasant voice - perhaps she will be pleasant to her lovers.
Radak: Tzor had great trade. People came to it from all nations and kingdoms with merchandise, like a Zonah, that many men come to her, and sometimes she is forgotten, and no one comes to her. So Tzor will be forgotten from the merchants who used to frequent it. After 70 years it will be visited like the Zonah's song (refer to 23:15:5:1). It will be rebuilt, and merchants will return to it.
Malbim: A Zonah gets a wage for her Zenus. So Tzor gets a price from strangers who enter its gates. Tzor is compared to a Zonah that her lovers forgot her for many days. They sing about her mockingly, that she should take a harp and go around in the markets and streets, in order that her lovers will remember her (16). So when Tzor wants to return to her status, it will send letters to all the nations around it, to remind them to come to it for trade. Scoffers and those who coin parables will sing the Zonah's song about her (refer to 23:16:1:1).
