It will likely run into the first few days of January. It did last year.
Conflicting reports of whether it finished Jan 5 or Jan 6, but either way-- I pulled Onatel during xmas last year, so I can personally verify it was during Xmas event, but in the month of January.
I thought that as well bit the calendar says something strange. Maybe the calendar was shorter than the event which is pretty unlikely compared to other spring events
I asked the Small Giant Staff about the planned dates for the Santa’s Challenge Seasonal Event, and got back an answer:
The event is planned to run December 1 08:00 UTC through January 4.
The Offers Calendar is scheduled to run only from December 1 through 31 — so the Event itself will extend beyond the Offers Calendar.
As the Event is planned to run into January, both the December HOTM (Grimble) and January HOTM (Vela) are expected to be available during the course of the Seasonal Summons.
Please note that as with all event dates, these are subject to change.