One way is to adapt the name phonetically (Kriistmas). This is slightly problematic because kriist is an actual dragon word meaning 'to stand'.

A few possible translations include Sulsesaviik 'day of the Savior' or Kiinsesaviik 'birth of the savior'. Sulsesaviik could potentially refer to Easter as well, so Kiinsesaviik may be the better option for clarity's sake.

To wish someone a Merry Christmas, you could then say Brit Kiinsesaviik! 'Beautiful Birth of the Savior!'