What does a grape farmer refer to when he uses that phrase?  A Christmas cluster is a cluster of generally green grapes which the farmer knows would not ripen – well, until Christmas.  That is, if Mother Nature could cooperate and make that happen.  As this photograph of Chambourcin grapes illustrates, sometimes a cluster of grapes is pollinized late – and therefore has no hope of ripening in time for the normal harvest.  Often grape farmers will cut these unseasonally green off the vines, so as not to impact the flavor of the remaining ones.