The custom is not to drink wine during the Nine Days, from Rosh Chodesh Av through Tisha Be-Av (really, midday of the Tenth of Av). The question is whether even a non-alcoholic beverage like grape juice is also not allowed. On the one hand, as I mentioned in a recent post (link), today grape juice is treated like wine in terms of its blessing and use for kiddush. So perhaps we can assume that it is considered wine for these purposes also.
On the other hand, it is the alcohol in wine that makes you happy. Maybe that is why it is not allowed during the Nine Days. If so, grape juice has no alcohol and should be allowed.
Click here for moreOr maybe the issue is that wine was brought as a libation in the Temple. During the Nine Days, when we mourn the destruction of the Temple, we refrain from drinking wine. If so, grape juice is cooked and could not be used in the Temple. Therefore, it should be allowed.
The Shulchan Arukh (Orach Chaim 551:10) states that even freshly made wine is forbidden. The Mishnah Berurah (ibid., 66) explains that even though it is sweet and weak, and could not be used as a libation in the Temple, it is still forbidden. Why? Because the community accepted as a custom to refrain from all types of wine. Since this weak beverage is still considered wine, we do not drink it during the Nine Days.
Based on this, the Nechemas Yisrael* (p. 114 n. 295) writes that we do not drink grape juice during the Nine Days. The Piskei Teshuvos (551:42) deduces the same ruling from the Mishnah Berurah.
R. Yisrael Ya’akov Fisher (Even Yisrael, vol. 9 p. 110 – link – PDF) uses the same logic to reach the same conclusion. He writes that we refrain from wine because it was used for libations in the Temple. However, the custom is to refrain from all wine, even that which was unfit for use in the Temple, including grape juice.
* By R. Yisrael Tanchum Dardak, published in 2005. I thank R. Moshe Schapiro of YU’s Mendel Gottesman Library for finding this source for me.