It's hard to answer questions about the meaning of a particular event or object in a given work. Such questions seem to rely on a one-to-one allegorical equivalence: a clean shirt means a soul unbesmirched by sin, twilight means approaching death, etc. But literature doesn't make meaning through a system where decoding a given token yields its intended significance.
Perhaps a more fruitful way to think about Christmas in In Memoriam is to ask: How do the repeated occurrences of Christmas work in the poem? What is their effect? When we compare the different representations of Christmas in the poem with each other, what similarities and differences do we see? What might we make of those parallels and contrasts?
Critics generally agree that the three Christmases in In Memoriam serve a structural purpose. Philip Mitchell writes:
It is possible to see the sequence structured around a three year period built around the three Christmas poems; each poem shows a growing sense of hope and resignation as time passes.
In other words, the Christmas poems do more than simply mark the passage of time. They also show how the poet's response to that death moves from despair and denial to acceptance and hope over the years following Hallam's death. This view goes back at least as far as 1883. In a work published that year, Tennyson's In Memoriam: Its purpose and its structure, John F. Genung observed:
It would seem, then, that the poem comprises different cycles of thought, to each of which Christmas, standing at the head, gives significance; and when we compare with one another the cycles thus introduced, we find that all have a similar structure, and that each contributes its part in an advancing and broadening series.
Genung argues that the first cycle focuses on the past: on memories of good times with Hallam that can never be relived. The second cycle focuses on the present: on the community of friends and on enjoyments that Hallam would have been a part of had he been alive. The final cycle focuses on the future: on the renewal of that community and its evolution into a higher force.
A reading of the three Christmas celebrations does demonstrate an overall trajectory from sorrow to resignation. In the first one, Section XXVIII, Tennyson describes his despair on hearing Christmas bells:
This year I slept and woke with pain,
I almost wish'd no more to wake,
And that my hold on life would break
Before I heard those bells again.
When the poet and his companions do attempt traditional Christmas activities, such as singing carols, those are followed by tears:
Then echo-like our voices rang;
We sung, tho' every eye was dim,
A merry song we sang with him
Last year: impetuously we sang:
We ceased: a gentler feeling crept
Upon us: surely rest is meet:
"They rest," we said, "their sleep is sweet,"
And silence follow'd, and we wept.
By the next Christmas celebration, Section LXXVIII, the mood has changed. The poet participates in the season's traditional joys again, but this time, those joys do not bring tears. Basically, the poet is all cried out:
Again our ancient games had place,
The mimic picture's breathing grace,
And dance and song and hoodman-blind.
Who show'd a token of distress?
No single tear, no mark of pain:
O sorrow, then can sorrow wane?
O grief, can grief be changed to less?
O last regret, regret can die!
No — mixt with all this mystic frame,
Her deep relations are the same,
But with long use her tears are dry.
The third Christmas, beginning at Section CIV, is different yet again. This time, the poet seeks to keep Christmas ("the night I loved") free of sorrow, while simultaneously abjuring all the traditional forms of celebration:
No more shall wayward grief abuse
The genial hour with mask and mime;
For change of place, like growth of time,
Has broke the bond of dying use.
Let cares that petty shadows cast,
By which our lives are chiefly proved,
A little spare the night I loved,
And hold it solemn to the past.
But let no footstep beat the floor,
Nor bowl of wassail mantle warm;
For who would keep an ancient form
Thro' which the spirit breathes no more?
Be neither song, nor game, nor feast;
Nor harp be touch'd, nor flute be blown...
The famous New Year poem that immediately follows underscores this commitment to letting go of empty grief as well as empty joys, and to a renewal:
Ring out, wild bells, to the wild sky,
The flying cloud, the frosty light:
The year is dying in the night;
Ring out, wild bells, and let him die.
Ring out the old, ring in the new,
Ring, happy bells, across the snow:
The year is going, let him go;
Ring out the false, ring in the true.
Ring out the grief that saps the mind,
For those that here we see no more;
"Let him die" and "let him go" refer to the year, of course; but they equally refer to Hallam. Tennyson is explicitly letting go of the grief of Hallam's death. This is not to say that he has gotten over it. Rather, as the next few poems show, he is determined to honor Hallam's qualities and remember him with joy rather than sorrow.
Structurally, then, the three Christmas celebrations are important milestones in the overall journey of In Memoriam from despair to acceptance, even to joy. The Prologue asks for "a beam in darkness: let it grow"; the Epilogue is an epithalamium for the poet's sister, who was engaged to Hallam at the time of his death, but is now the bride of another. As the joyful Epilogue shows, the poem as a whole shows that the prayer in the Prologue is answered:
help thy foolish ones to bear;
Help thy vain worlds to bear thy light.
The Christmas sections show the different stages through which "foolish ones" progress as they learn how to cope with sorrow and arrive at the "light".