The title points to the winter solstice, the longest night of the year, and the poem stays inside that darkness. A single candle flickers on frost, and the speaker, looking at it, is visited by what they call "the old melancholy vision": a familiar feeling that time is running out, that the cupboard holds nothing left for tomorrow. The image is homely and a little bleak. It makes the future sound like a pantry that has been emptied.
Then the last two lines turn. "One hard hope" goes "passing by on foot," like a stranger on a cold road. It is not a comforting or radiant hope, only a stubborn one, and it doesn't stop to stay. The mood is quiet, wintry and tired, but not hopeless. The speaker seems to recognize this old gloom as something that returns, and also notices that something small and resilient keeps passing through it, much like the candle against the frost.