"My November Guest" begins with the speaker personifying their "sorrow" as a woman who has come to visit. Treating sorrow as a "guest" implies that the speaker isn't always sad (and that this "sorrow" is something separate from the speaker rather than an inherent part of who they are). When this sorrow does swing by, however, "she" makes the speaker see the world differently.
That's because the speaker's sorrow "Thinks these dark days of autumn rain / Are as beautiful as days can be." In other words, the speaker thinks that the gloomy, rainy days of fall are just as lovely as anything else. Diacope (the repetition of the word "days") creates a pleasing rhythm while simultaneously emphasizing the fact that these "days" are both "dark" and "beautiful." The pounding /d/ alliteration in "dark days" adds emphasis to this phrase, which describes both the weather and the "dark[ness]" of the speaker's thoughts and feelings. It's as if the countryside is mirroring the speaker's inner landscape.
The poem is written in iambic tetrameter. This means that each line contains four iambs, poetic feet made up of an unstressed syllable followed by a stressed syllable: da-DUM. Here's the first line, for example:
My sor- | row, when | she's here | with me,
The meter stays very regular throughout the poem, lending it a familiar, predictable rhythm. This steadiness, in turn, suggests both the understated loveliness of the season and the speaker's thoughtful, contemplative state of mind.
That said, there is a minor variation on this meter in line 2:
Thinks these | dark days | of aut- | umn rain
By replacing the second iamb with a spondee (two stressed syllables in a row), Frost ensures that the phrase "dark days" stands out even more to the reader's ear.