At the beginner jams held in December, you are welcome you to call Christmas/seasonal songs at the jam that you would like to play that you believe would be a good 'fit' for the beginner jam group. And you need not wait until the second half of the evening to call these.
Of the many Christmas/seasonal songs that have been played in past years at the December through early January beginner jams, the ones that I think have worked the best in the context of the beginner jam are:
Auld Lang Syne
Away In A Manger
Good King Wenceslas
Shepherds In The Field
but I think that some other good song choices for the phase that the beginner jam is currently in might include:
Go Tell It On The Mountain
Joy To The World
In the attachments, I have included the chord progressions I usually use for each of these 9 songs.
Song of the week write-ups, complete with recordings and melody sheets, for Auld Lang Syne, Away In A Manger, Good King Wenceslas, and Silent Night can be found here:
All of Jason's Songs - Idaho Bluegrass Association
In addition to the jam videos available here Jason’s YouTube Links – Alphabetical Listing – Parisology (cyberplasm.com) for Away In A Manger, Good King Wenceslas, and Silent Night, I have also made a jam video that includes Jingle Bells, Shepherds In The Field, and Auld Lang Syne (Beginner Bluegrass Jam 12/12/2020 | Jam Along | Idaho Bluegrass Association | Jason Homey - YouTube), and a jam video that starts with Joy To The World Intermediate Bluegrass Jam 12/12/2020 | Jam Along | Idaho Bluegrass Association | Jason Homey - YouTube
Finally, here is a live performance from the December 2019 IBA open mic of 6 Christmas/Seasonal songs (O Little Town Of Bethlehem, Away In A Manger, Good King Wenceslas, Silent Night, Shepherds In The Field, and Auld Lang Syne) by Jason Homey & The Snake River Boys:
Jason Homey and the Snake River Boys, IBA Open Mic, 12_16_19 - YouTube