You will have to instruct the LINQ query that the local part should be done locally:
var tariheGoreGrupla = (from hatirlatma in
(from hatirlatmaX in toDoItemsInDB
orderby hatirlatmaX.Due_Date
select hatirlatmaX).AsEnumerable()
let gun = GetDate(hatirlatma as ToDoItem)
group hatirlatma by gun into tariheGore);
AsEnumerable()
means "from this point on, use the local enumerable methods and don't try to convert this into an SQL query".