The title points to the winter solstice, the darkest stretch of the year, and the speaker answers it in an unusual way. Instead of huddling indoors or brooding, they decide to become a "disciple" of the night, as if darkness were a teacher worth following. What they want to learn is small and specific: how things are in the world of the mole, a creature that lives underground and barely needs light.
The last line, "never mind my heart," is where the feeling sits. It suggests the speaker has troubles or sadness they'd rather not look at, and that studying the mole is a way of stepping outside themselves. Judging by the tone, though, this isn't bitter. It's quiet and a little wry, even humble. The mood is hushed, dark and inward, but also curious. The poem seems to say that in the deepest part of the year, paying close attention to another creature's hidden life can be a kind of comfort.