Late winter is the setting here, when the days get longer but the weather stays harsh. The speaker notices a odd truth: the cold deepens at the same time as the light strengthens. "Apricity," an old word for the warmth of the sun on a freezing day, captures that mix perfectly.
The second stanza turns to a young oak that held onto its leaves through the fall. It stands rustling quietly beside its older "mother" tree, and the speaker imagines light being stored up and feeding its roots. The small tree seems protected and patient, gathering strength out of sight.
The last two lines step back to include us all: "We live these days / in a haven of possibility." The mood is calm, hopeful, and gently watchful. Nothing has bloomed yet, but the speaker feels that something is about to, and finds comfort in that waiting rather than impatience.