This particular section of this wonderful poem, in which Tennyson explores his response to the loss of his best friend, Arthur Hallam, describes the first Christmas after Hallam's death. However much the speaker and those with him try to be happy and merry, the absence of Hallam is something that makes this impossible:
A rainy cloud possess'd the earth,
And sadly fell our Christmas-eve.
In addition, the "winds" sweep "the winter land." This is an example of pathetic fallacy, where the weather enacts the emotions of the people. Here, the speaker and his friends are feeling depressed and grief-stricken because of the "shadow" that afflicts them all: the absence of Hallam. The images of nature here therefore symbolise the grief, loss and oppression that such an absence brings.
However, by the end of this section, after the speaker has reminded both himself and his audience about the promise that is held in the Christian gospel of eternal life, he is able to present nature in a more positive way, as representing this Christian promise:
Rise, happy morn, rise, holy morn,
Draw forth the cheerful day from night:
O Father, touch the east, and light
The light that shone when Hope was born.
Christmas day itself, and in particular the "cheerful day" is shown to symbolise the newfound hope and consolation the speaker finds in the thought of Hallam having attained eternal life.