Forefathers' Eve is a secondary quest in The Witcher 3: Wild Hunt. After completing A Towerful of Mice, the Pellar will contact you at one of two possible locations, depending on whether the quest Family Matters has already been completed or not. If it was, the Pellar will be outside Keira's hut, otherwise he will contact you at Crow's Perch near Fergus Graem's forge. In any case, the Pellar will ask you to help protect him and a group of villagers during the Forefathers' Eve ritual on Fyke Isle. You must meet him at the stone circle on the isle at midnight, so if it isn't quite time, meditate to progress the quest.