It is through A looking glass. I think really it is a reference to the flash FORWARD. Do we see THE future or A future.

'Are these the shadows of the things that Will be, or are they shadows of the things that May be only?'

So the Spirit of Christmas Future asks Scrooge. Dickens is another author frequently referenced.

Besides the date of the episode is on or about Christmas eve.

In addition the title with THROUGH is probably a reference to 1 Corinthians 13:12:

12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.