This excellent short story is rich in sensory detail which creates images, which, in turn, create the mood of the story, or the emotions that the writer wants us to feel. When considering your question you will want to think about these issues - how does the author create mood in his story? One of the most poignant aspects here is the juxtaposition or the placing next to each other of memories of extreme happiness with indications of the future sadness that time will bring. Consider this example:
The wind is blowing, and nothing will do till we've run to a pasture below the house where Queenie has scooted to bury her bone (and where, a winter hence, Queenie will be buried, too.) There, plunging through the healthy, waist-high grass, we unreel our kites, feel them twitching at the string like skyfish as they swim into the wind. Satisfied, sun-warmed, we sprawl in the grass and peel satsumas and watch our kites cavort. Soon I forget the socks and hand-me-down sweater. I'm as happy as if we'd already won the fifty-thousand-dollar Grand Prize in that coffee-naming context.
Note how this passage creates a mood of absolute nostalgia as the narrator looks back to his childhood Christmas memory as an adult and remembers all the happy amazing moments - the feeling of the sun on his skin, the tug of the kites on their hands and the emotion of happiness. However, the relentless passing of time is also alluded to, and Queenie's mortality adds a discordant, sad note to the passage.
Of course, if you want to find more examples of sadness or of deep loss, consider this passage:
Life separates us. Those Who Know Best decide that I belong in a military school. And so follows a miserable succession of bugle-blowing prisons, grim reveille-ridden summer camps. I have a new home too. But it doesn't count. Home is where my friend is, and there I never go.
Here the way that time "separates" us and takes us away from our "home" is sadly evoked, as descriptions such a "bugle-blowing prisons" clearly show us how the narrator feels about his separation. Nostalgia, it seems, is just for a specific period of our childhood - we will always look back, cherish and ponder those golden memories even as the mortality of life pushes us ever-more away from them.