Here's this year's version of a tradition that goes back to 1972.

The Post Office began to use cancelled pictorial indicia in 2006 (using the Christmas secular images).
Since 2013, they have used the Picture Potage format.
This is a fascinating little corner of the stationery field of philately. They are pretty hard to find - even the latest - since postal elves keep this project top secret. (They are all numbered & registered to prevent fraud and mayhem.)
According to Tony Shaman (catalogue creator), over a million letters are sent out. Congratulations and thanks go to over 11,000 postal elves. Yet - few are ever seen for sale.
Some collectors write fictitious letters to acquire the envelopes, but if discovered (which is highly likely since "he sees you when you're sleeping..."), they will end up on the "naughty" list and receive next year's letter with this indicia:

I have found that a box of chocolates will distract a postal elf long enough to allow an envelope or two to go missing.
All the best to you all for the new year.
[PS If anyone has an extra 1988 (Big Santa cancel) envelope, I would be glad to hear from you.]