The Feast of the First Fruits of Wine is a holiday celebrated by the ancient Israelites as purported in the Temple Scroll of the Dead Sea Scrolls. The holiday, which is observed on the third day of the fifth month (Av), is not mentioned in the Bible. The date is significant because it is exactly 50 days after the 15th of Sivan, the festival of Weeks.
On this holiday, the Israelites were commanded to bring their first fruits of the grapes to the Temple as an offering. This holiday was a very happy day because "the wine gets atonement on this day" and was commanded to be observed as an annual celebration perpetually. After this holiday, the children of Israel were permitted to eat from their grape vines, whether the grapes were ripe or still unripe, because after the observances of this holiday, they had made a full atonement for the wine. It so happens that this day coincides with the modern Jewish period called "The Nine Days", a time when mourning practices take place and when wine and meat are prohibited.
This holiday is not observed by any known modern Jewish group today.