It is flown during the winter months every year.
The tasks are posted here every month, and you fly at your local flying field.
You must upload your scores yourself here.
So Have Fun and Just Fly!
Correction: actually, the winterpostal went much further back. It was managed by several others, like Loet Wakkerman, the legendary creator of the Tantalus.
I think it was started in 2006/2007 by Theo.
Please check here for further details.