Hello,
The sign in event should activate at midnight on the 21st at Pacific Standard Time (LA region time). It may be showing as active/available due to your current system settings, but we have been told that everything will go live at the above listed time.
Thanks, and sorry for the inconvenience!

Any updates about this? It's already Dec 22, but I still experience the same problem.

Hello,
As far as I am aware, the sign in portion of the event is proceeding fine. I've had many reports of people being able to successfully sign in at this point. You should not be getting the Original Poster's error message at this point. if you are, then you will need to open up a customer support ticket in game to see what the issue is. Here is the link.