Well the days of Saturnalia run across Christmas time, and at least the narrative or mythology of Saturnalia includes masters serving meals to their slaves.
Of course this would be ritualized and only happen I guess in an orderly household situation ...
we aren't talking the kind of vision of slavery here, that has the slaves constantly kept in shackles and chains and whipped all the time.
So a little bit of ritualized inversion might be a thing.
Another idea is you can have a threatening object of punishment
(a grisly whip, a noose, a set of crux-nails, a crown of thorns ... whatever you wish)
which is individually made for each slave at the beginning of the year,
and perhaps kept on a wall beside their name tag as a constant reminder of a possible fate.
Towards the end of the year, if there has been no need to use it,
that object of punishment is ceremonially destroyed,
as a reward for a year of good service.
And during the inbetween-days when the new year is born after the solstice,
briefly that threat does not hang over them symbolically,
and with the new year, it is made again.
If you're actually thinking of a "slave christmas" in a worshiping Christian environment, that tries to gather masters and slaves in one church (instead of having masters and slaves basically going about their own worship) then such a ritual would try to justify the role of the masters and try to make them appear as intermediates for God's authority or something, so just heaping on additional punishment wouldn't cut it.