U is

 difficult! 

When it comes to Christmas words I found seeking words beginning with U to be an Unpropitious aim.  Urged on be my determination to give no-one chance to take Umbrage at my failure to post a full advent calendar, I tried to Use my brain.  It is an Uneasy exercise.

Unity would be ideal, with all people being Unified for the common good.  Ultimately.  Unwrapping Unity from the parcels Under my tree would be wonderful.  It would truly make this Christmas Unique.  

(Sorry, don't know what the posh coves are doing today as Holly Dog needs me for day care.)