When is that day?
Rashi: It is the time of Ge'ulah.
What is the significance of saying "Kerem Chemer"?
Rashi: They will sing to Yisrael, this is the vineyard of strong wine, which produced good wine. Malbim - above (Perek 5), Dodi sang about the bad grapes that they produce.
Radak: Initially, Yisrael were compared to a vineyard that made Be'ushim (bad grapes, or a species of bad grapes - 5:2, 4). Now it says that at the time of salvation, they will be called a vineyard that makes good grapes that produce esteemed, quality red wine. Chemer is redness. The esteem of wine is based on its redness - "Al Tere Yayin Ki Yis'adam" (Mishlei 23:31). The Targum of wine is Chamra.
Malbim: Kerem Hashem, i.e. Yisrael, not only will they produce grapes - they will give wine in abundance.
