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Letters from Father Christmas always fascinates me because it combines Tolkien's affinity for storytelling, letter writing, world building, and the intention to give his children the kind of childhood he never had.
A Christmas thread!
A Christmas thread!
Between 1920 and 1942, Tolkien kept a yearly tradition of writing letters to his children every Christmas, making them look like they came from a figure called "Father Christmas". He even created his own post stamps and envelopes to make them look real.
Every letter told adventures and misadventures of several characters: Father Christmas, an Elf secretary called Ilbereth, the North Polar Bear, and two sidekick bear cubs Paksu and Valkotukka (Finnish for "Fat" and "White Hair").
Tolkien also added his own illustrations. I have to apologize but a lot of my favorites seem to include the Polar Bear falling for some reasons, like when he's walking down the stairs with presents and slipped on a soap bar, or when he's fixing the roof and falling from it. 
Just like how he described his Middle-earth legendarium, his description of Father Christmas' world was evocative and deep. For example, in one letter, he described in detail a ancient Goblin Cave and the painting on its walls (pictured).
We also see traces of the world Tolkien was working on (The Silmarillion). For example, his secretary Ilbereth reminds you of Elbereth, his Merry Christmas messages were written in letters that look like Tengwar system, the Arktik language holds traces of Quenya, and many more.
Many letters referred to real situations. The Polar Bear teaching the kids how to hold a kitten. The 1939 letter referred to World War II with this: "I am very busy and things are very difficult this year owing to this horrible war. Many of my messengers have never come back."
Father Christmas' last letter came when his youngest child turned 15, with a bit of melancholic tone: "I suppose after this year you will not be hanging your stocking any more. I shall have to say goodbye.... We always keep the names of our old friends and their letters..."
The letters were edited by Baillie Tolkien, the wife of Christopher Tolkien, and published in 1976. Like Christopher, Baillie is familiar with Tolkien's works, previously working as his secretary and compiling index to Songs and Verses in the 2nd edition of The Lord of the Rings.
I also made a post about Letters from Father Christmas in Indonesian four years ago. Fire up your translation tool of course if you don't read in Indonesian! (I have edited it so it will be easier to read after translated). https://putri2wotan.wordpress.com/2016/12/24/keajaiban-renungan-letters-from-father-christmas-tolkien-natal/?preview=true
