I've written before on Tolkien's treatment of goodness. Tonight I'd like to glance at the other side of the coin: Tolkien's acknowledgment of evil. The evil wizards and the orcs and the explicitly evil elements are obvious enough in his works. But in Tolkien's writing, unlike many heroic epics, the heroes do not all simply live happily ever after. In Tolkien's reckoning, on October 6 of each year Frodo was visited by returning pain, likewise in March of each year, on the anniversaries of serious wounds he had taken. When Frodo returns to the Shire, he cannot take up his life again. Something similar had happened long years before to Celebrian, who was Elrond's wife, Galadriel's daughter, and Arwen's mother.
In Tolkien's earlier work, The Hobbit, Bilbo's adventures stay with him after his return, though not in a bad way at first. Bilbo spends much of the rest of his days in his study, writing, while the evil he has unwittingly brought back with him slowly grows on him. Frodo's darker adventures leave him at times haunted, and again after his return he spends much of his time in his study, writing.
As for Tolkien, after he returned from his own adventures in the Great War / World War I, we all know where and how he spent much of his time: in his study, writing. I've made a few searches and haven't found what I was looking for. But I'd be very curious if anything had happened to Tolkien on the dates he chose for Frodo's yearly visits of pain: March 13 and October 6.