 In Eastern European Ashkenazi Jewish tradition , vegetarian borscht served with sour cream and boiled potatoes on the side , known as <unk> borsht , is considered an essential dish during the Passover period . As the holiday is observed in spring ( March or April ) , the preparation of Passover borscht used to provide an opportunity to use up the beet sour left over from pickled beetroots that had been consumed during winter , remaining potatoes that had been stored throughout the winter and sour cream that was readily available in the new calving season . Cold borscht blended with sour cream is also popular on Shavuot ( Feast of Weeks ) , a holiday customarily associated with dairy foods , observed in late May or early June . <unk> <unk> , or the third meal of the Shabbat , often includes borscht as well . 
