The gemara clearly says that wine fresh from the press - that is, grape juice - is permitted to drink even at the seuda hamafsekes on erev Tisha B'Av!
Yet the custom today is to forbid grape juice, and some even forbid grape juice flavoring the entire 9 days.
There is also a question if only pure grape juice, like the case in the gemara, is forbidden, or even if it is mixed with water or other juices, or from a concentrate.
M K, Brooklyn, NY
The Shulchan Aruch records the custom to forbid wine straight from the press on Erev Tish'ah b'Av (1). Even though the Shulchan Aruch does not forbid such wine during the week of Tish'ah b'Av, other opinions do (2), which practically for Ashkenazim would mean starting from Rosh Chodesh Av (3).
1. See the comment of the Gra here also https://www.sefaria.org.il/Shulchan_Arukh%2C_Orach_Chayim.552.2?lang=bi&with=Beur%20HaGra&lang2=en
2. See Mishnah Berurah and Gra here https://www.sefaria.org.il/Shulchan_Arukh%2C_Orach_Chayim.551.10?lang=he&p2=Mishnah_Berurah.551.66&lang2=he&p3=Beur_HaGra_on_Shulchan_Arukh%2C_Orach_Chayim.551.10.2&lang3=he