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.