O Christmas Kudzu
A little while ago, offensive_mango pointed to some delightful Humanist Christmas Carols. Silent Night is replaced with Be a Bright, The First Noel with There Is No Hell, the faithful are urged to come from Bethlehem and "ignore him", while, perhaps most charmingly, God Rest Ye Merry, Gentlemen becomes

The World of Richard Dawkins
Is a place where you will find
A scientific plethora
To stimulate your mind.
Expand your intellectual side;
Leave ignorance behind.
O, tidings of knowledge and truth,
Knowledge and truth.
O, tidings of knowledge and truth.

We atheists will have no truck with comfort and joy, though apparently we will have truck with personifying pine trees and then settling our consciences by instead buying fakes made from non-renewable fossil fuels which used to be, er, dead trees:

O Christmas Tree, O Christmas Tree,
How murderous and wasteful
To end your life so carelessly,
So thoughtless and distasteful.
[...]
I'll work for change, I promise thee,
For only Man can save a tree.
Long life to thee, O Christmas Tree,
I'm buying "artificial."

Revising lyrics to make them more scientific isn't a new idea. Babbage famously wrote to Tennyson complaining about the line "Every minute dies a man, every minute one is born":

If this were true the population of the world would be at a standstill. In truth, the rate of birth is slightly in excess of that of death. I would suggest "Every moment dies a man, every moment 1 1/16 is born." Strictly speaking the actual figure is so long I cannot get it into a line, but I believe the figure 1 1/16 will be sufficiently accurate for poetry.

Furthermore, Christmas songs have notoriously fickle lyrics. Hark! the herald angels sing began as Hark! how all the welkin rings. Have Yourself a Merry Little Christmas originally opened "Have yourself a merry little Christmas; it may be your last" before being revised to its familiar form, and eventually to "Have Yourself A Blessed Little Christmas".

Since people are so keen to rewrite Christmas songs, then, I present my Christmas Song Generator, whose usefulness is rivalled only by the shoddiness of its decorative holly. At the press of a button, you can make carols appropriate for atheists:

Bring me flesh, and bring me Richard Dawkins,
Bring me pine logs hither.

You can make secular songs appropriate for Christians:

Rudolph the red-nosed angel
Had a very shiny nose.

And you can make everything appropriate for me:

And so this is Xmas,
I hope you have bees.