“In drear nighted December” begins in the darkest part of the year. It's not just winter, but December in particular, the month of the longest night—a “drear,” gloomy, dark time indeed. Out of the midst of this darkness, the poem’s speaker makes a wistful apostrophe to a leafless tree:
Too happy, happy tree
Thy Branches ne'er remember
Their green felicity—
This tree, in other words, is much luckier than it knows. (“Happy” here means “fortunate” as much as “cheerful.”) It might be frozen stiff—but that just doesn’t bother it. Why? Because it has no memory. It can’t recall the “green felicity”—the leafy joy—of summer. Losing its leaves thus makes no difference to these “branches”; whatever consciousness they had of their leaves fell away when the leaves did.
To the speaker, this “happy, happy” forgetfulness—and notice the yearning epizeuxis—sounds wonderful. But there’s an implied juxtaposition here: clearly, the speaker enjoys no such lucky oblivion when they lose something. This will be a poem, not just about grief, but about the particular pain of having, losing, and remembering.
Winter is an ancient symbol for death, grief, and loss, and the speaker will draw on all those old ideas here. But rather than reaching for the good old pathetic fallacy, personifying nature and using an iced-over tree as a symbol for an iced-over heart, this poem's speaker will insist that the world's seasons and the heart's seasons don't work in at all the same way.